Probably this. The activity took place a while back and is beyond the typical DDF guy racking up points with Amex
I'm aware - this is just a paraphrasing of the indictment; it's lacking any real details. There's plenty of shtick going on out there in MS land that you may not be aware of.
MS has always operated in a grey area; if Amex was motivated enough they could get the government to charge most MSers with some sort of bank/mail/wire fraud. These are very broad laws.
I'm not aware of any law stating that you can't open a merchant account at a card processing company and swiping even your own cards. Other than Amex getting upset and saying that you're fraudulently trying to obtain bonus points, and referring you to the DA.