More of a terms of service violation. In my experience growing up in Las Vegas they really only arrested folks who used some device to physically cheat. People who worked the odds by card counting or perhaps taking advantage of an out of balance roulette wheel generally were simply banned for life (which technology and facial recognition makes much easier today apparently).

Conspiring with others to break terms of service is of course a crime, and doing so across state lines, is its own sort of nastyness. But you can see from the prison terms (2 years) it wasn't exactly a big threat to these guys.