It's not about the price per se, but a consequence of respecting the freedom of your users. You may or may not believe that people have a right to study, modify and share software, but some people do. Software freedom is like freedom of speech; you can't take it away from people by contract just because it's convenient for you to do so.
... That's a perfectly valid point of view. But it's not the only point of view. Clearly, there are a lot of people that don't, in practice, support it.