Companies who do 24/7 really have nothing to worry about despite the small stack of letters from the FAA saying it's illegal.
A) as previously mentioned, no pilot is going to rat them out because by doing so you're admitting you took an assignment you knew was legal. Even if you had the cojones to do that, you'd likely end up with at least a suspension, and the company would get a slap on the wrist and continue doing it.
B) the POIs of these companies may as well be in bed with them as they continue to tolerate it. Make no mistake, there are some great POIs out there, but there are also many who let a LOT slide at their carriers for a variety of reasons. Until a company has a POI who actually forces them to, you know, follow the law, they have zero incentive to not take advantage of their crews.
C) management of these carriers seems to almost delight in telling pilots they'll be on call 24/7. They'll explain how "everyone does it" and "this business wouldn't be competitive with the normal rest rules". A lot of young and/or eager pilots will happily jump at the opportunity, maybe thinking "well hell, I'll be making money on a trip while Brand X is on rest!", unfortunately contributing to the problem. If you're a member at PPW there's a member who actively recruits in damn near every thread for pilots for a certain Michigan-based carrier who utilizes 24/7 on call. I think the guy has pulled out every excuse I can imagine to justify it, and I think what's worse is he actually believes a lot of it.