Disclaimer: I am not an attorney: contact a licensed law professional in your area before making a decision with legal ramifications.

“[T]o pay one sales person based on a commission structure and not pay another salesperson based on a commission structure… would only be illegal if you were distinguishing between them based on a category you’re not allowed to, such as race, sex, one of them being over 40, etc.”

The thing is, as soon as the pay discrepancy is detected the first instinct of anyone with any legal inclinations would be to identify any sort of protected categorical differences between the two individuals. The second instinct would be to look for any indication that there is some plausible correlation. And this is easier to achieve than you might think, because in the United States discrimination is a civil matter and not a criminal one (unless there is some sort of threat, violence or other clearly criminal object involved). So if the matter ends up in court the standard of proof will be proponderance of evidence, i.e., whether it is more likely than not that there was unlawful discrimination (the “disparate impact” test might also be used). This is easier to demonstrate than to prove “guilt beyond a reasonable doubt.”

So employers do need to tread VERY carefully on these questions.

This is definitely not true in the U.S. It would only be illegal if you were distinguishing between them based on a category you’re not allowed to, such as race, sex, one of them being over 40, etc.