Well, it's not a crime, and nobody's going to get arrested
So yes, it's "legal."
Do you mean "is it a breach of contract?" Because the TTS is listed as an experimental feature, I would guess probably not. You'd need to dissect the ToS.
In practical terms, there may be some changes in the immediate future, as current titles are updated, and I'd say that Amazon would be smart to honor a refund request from any user that bought a book that disabled its TTS status. But 6 months from now, I doubt this will be an issue -- I doubt publishers will be changing back and forth willy-nilly. Hopefully, Amazon will say on the book's page if TTS is disabled before you buy.
And there's always MobiDeDrm as a last resort.