I would think it has more to do with legal responsibility. They encrypt the movies with CSS to show that they are not simply putting the material out into the public without doing something to protect it. Much in the same way that you don't leave your front door open, as that might be seen as an invitation to come in.
Since decryption of movies is seen as illegal, according to the DMCA in the US, they are making sure that anyone who distributes their content without permission cannot access it without breaking that encryption, and thus are guilty of both illegal distribution and a DMCA violation. Of course you can copy a movie as it plays and make an inferior reproduction, but that won't be acceptable by most people looking for copies on the net.
Decryption for personal use and fair use of purchased material is another area altogether, though the movie studios will argue against this as well.
What I don't really understand is the more advanced protection schemes that they continue to come out with. Those are broken quickly and do not delay release of the materials onto the net by enough time to matter. I can't see that it helps drive sales at all.