And how many of you download films and music that you actually pay for?..

Not quite, but similar.
You can't use a service and expect it to be completely free. Very little in life is completely free, you pay for apps either with a fee or by adverts. Or in this case by agreeing to them having rights to your photos.
I pay for my Flickr account, because a) I want the pro account features and b) so I have full control over my photos. If I want to licence a photo for someone to use, then I can.