It depends on the movie. Some are made free because the film company allowed them to be free because of some sort of cross-promotion deal. Other times, they're just illegal.
It depends. If the people that made the movie release it on to the internet themselves and declare that it's for free, then yes. If it is a movie that had been ripped off of a DVD by a third party user and released onto the internet then no.
As long as they are acquired through legal channels and the website distributing them has permission from the films owners then yes they should be completely legal assuming that you meet age requirements.
