How often do you read the Terms of Service or End User License Agreement when you set up an account on a web service (e-mail, social networking, etc.) or when you install software? My guess is: not that often. Well, you'd be amazed at what you're agreeing to when you go through those systems. Those are the rules by which the company agrees to abide, and the rules by which they expect you to abide in using their service.
I've seen people get billed for the use of a service that they thought was free because that aspect of the Terms of Service wasn't clear to them. That sucks, but is it worse than giving up rights to your content permanently? Maybe being a photographer makes me more keenly aware of the issues of the latter. In any case, I'm not the only person with a concern.
Facebook recently changed it's Terms of Service, claiming perpetual rights to content that was posted. This caused an uprising of users around the world. Many saw it as an effort to claim ownership over the what users posted. Photographers who make a living from selling their photos were justifiably worried. Well, the uproar has borne fruit. Facebook responded both to explain their intentions (to have the right to use the content to help you share it) and to revert to their old terms of service.
Still, if people hadn't reacted so strongly, most people may not have noted what happened at all. As conspiratorial as it may sound, to ensure our rights are protected, we must remain ever vigilant.