Foreign generics are illegal to import in the US because they violate patents held by US companies. Advertising an illegal import is one thing, but it's unclear how legal it would be to advertise something like "Come visit India where you can get generic Singulair while you're here!"
That said, dentists in Mexico aren't necessarily violating any US laws by doing business there, and you can't import dental work either.
Exactly. It's illegal to import those drugs. It's not illegal to go to another country to get treated.
I'm not a lawyer, and this isn't legal advice, but I'd imagine the only way they could stop you is by passing a new law, and it would be very controversial to try. Existing legislation (and stuff that can be done within the existing framework) is less controversial - bureaucrats and politicians will just say "it's the law, and I may not personally agree with it, but that's just the way things work". You can't say that if you are introducing a new law.
That said, dentists in Mexico aren't necessarily violating any US laws by doing business there, and you can't import dental work either.