This is probably true. Whenever I have published articles (I am a mathematician), I have been asked to sign some long agreement which I only skimmed. We have some rights to publish and distribute our work... but not unlimited... something complicated...
In practice, I just put copies of all of my papers on my website where anyone can download them. I have many colleagues who do the same, and many colleagues who are more scrupulous about following the journals' rules. In any case, I don't know of any scholars who have gotten into trouble for distributing their own work.