Justice Gorsuch's dissent in Gundy v. United States relies on Amity Shlaes's book The Forgotten Man for this description of some of the facts in the Schechter cases: "Kosher butchers such as the Schechters had a hard time following these rules. Yet the government apparently singled out the Schechters as a test case." According to Wikipedia (which on this question is accurate), Shlaes is "an American author and newspaper and magazine columnist ... [who] writes about politics and economics from a US libertarian perspective." Her description of the Schechter case is consistent with that description of her perspective, and (depending on what Justice Gorsuch had in mind when referring to the difficulties kosher butchers had) almost certainly inaccurate.
One of the requirements the Schechters violated was a "straight killing" rule, under which buyers had to purchase all the chickens in a coop (or half coop) after chickens unfit for consumption had been removed. Shlaes writes that "to suggest ... that Schechter chickens were unfit was ... to suggest that their kosher slaughterhouse was not really kosher," because, she suggests, under Jewish law "[c]ustomers ... had the right to choose their birds, and this in turn ensured that everyone involved had a chance to determine whether the product was as healthy as possible." (I write "suggests" because Shlaes doesn't lay out the argument she appears to be making, but I can't figure out anything else that she could mean by "not really kosher.")
Shlaes provides no citation to, or discussion of, the applicable Jewish law, and according to one academic expert in the field I consulted, nothing in the law of ritual slaughter appears to require that customers as well as the sellers' employees who qualified as slaughterers for purposes of Jewish law have the right to inspect chickens before sale. Nor, as far as I know after reading the trial transcript, did the Schechters ever claim in court that they had violated the straight-killing requirement because of their view of the requirements of Jewish law. (Once the Schechters allowed customers to pick out scrawny chickens, the Schechters sold those chickens to "the colored trade," as one witness at the trial put it.)
There's more to the story about the Schechters. They weren't a corner butcher shop, but the largest slaughterhouse in Brooklyn, processing more than 34,000 pounds of chicken in a typical week, and they earned more than $1,000,000 dollars per year (over $15.3 million in 2019 dollars). Nor were they completely honest businesspersons. They clearly did sell chickens infected with respiratory illnesses, which might have included tuberculosis. (Observing that a live chicken had a respiratory ailment was relatively easy, though determining what the illness was, was difficult. And once the chicken had been slaughtered the ultimate purchaser had no way of knowing that chicken had such an ailment.)
As an aside, the government didn't "single out" the Schechters as a test case; government lawyers had brought cases against what they thought were much better candidates for test cases, but those cases weren't in a good litigation posture when the Schechters lost in the lower courts and as a result could petition the Supreme Court for review. For example, the government dismissed its appeal from a lower court decision dismissing the indictment of the owner of several large sawmills (who had refused to comply with the applicable code's wage-and-hour requirements) because the "record" -- actually, only the indictment -- hadn't been framed with an eye to the non-delegation doctrine, which assumed its importance after the appeal had been filed.
In short, the journalist in Shlaes appears to have led her to make up a good story -- and Justice Gorsuch, a participant in her epistemic community rather than in the epistemic community of historians, found the story credible enough to hint at in his opinion.
One of the requirements the Schechters violated was a "straight killing" rule, under which buyers had to purchase all the chickens in a coop (or half coop) after chickens unfit for consumption had been removed. Shlaes writes that "to suggest ... that Schechter chickens were unfit was ... to suggest that their kosher slaughterhouse was not really kosher," because, she suggests, under Jewish law "[c]ustomers ... had the right to choose their birds, and this in turn ensured that everyone involved had a chance to determine whether the product was as healthy as possible." (I write "suggests" because Shlaes doesn't lay out the argument she appears to be making, but I can't figure out anything else that she could mean by "not really kosher.")
Shlaes provides no citation to, or discussion of, the applicable Jewish law, and according to one academic expert in the field I consulted, nothing in the law of ritual slaughter appears to require that customers as well as the sellers' employees who qualified as slaughterers for purposes of Jewish law have the right to inspect chickens before sale. Nor, as far as I know after reading the trial transcript, did the Schechters ever claim in court that they had violated the straight-killing requirement because of their view of the requirements of Jewish law. (Once the Schechters allowed customers to pick out scrawny chickens, the Schechters sold those chickens to "the colored trade," as one witness at the trial put it.)
There's more to the story about the Schechters. They weren't a corner butcher shop, but the largest slaughterhouse in Brooklyn, processing more than 34,000 pounds of chicken in a typical week, and they earned more than $1,000,000 dollars per year (over $15.3 million in 2019 dollars). Nor were they completely honest businesspersons. They clearly did sell chickens infected with respiratory illnesses, which might have included tuberculosis. (Observing that a live chicken had a respiratory ailment was relatively easy, though determining what the illness was, was difficult. And once the chicken had been slaughtered the ultimate purchaser had no way of knowing that chicken had such an ailment.)
As an aside, the government didn't "single out" the Schechters as a test case; government lawyers had brought cases against what they thought were much better candidates for test cases, but those cases weren't in a good litigation posture when the Schechters lost in the lower courts and as a result could petition the Supreme Court for review. For example, the government dismissed its appeal from a lower court decision dismissing the indictment of the owner of several large sawmills (who had refused to comply with the applicable code's wage-and-hour requirements) because the "record" -- actually, only the indictment -- hadn't been framed with an eye to the non-delegation doctrine, which assumed its importance after the appeal had been filed.
In short, the journalist in Shlaes appears to have led her to make up a good story -- and Justice Gorsuch, a participant in her epistemic community rather than in the epistemic community of historians, found the story credible enough to hint at in his opinion.