Well, it reads in the terms of Twitter:
You retain your rights to any Content you submit, post or display on or through the Services. By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).And that's almost the same that Facebook stated in their terms. Why are they doing such a thing? Are they evil™ ? Okay first of all, I'm not a lawyer. But maybe I can get some light into this.
Why terms of usage at all?
For a company that operates worldwide, it's necessary to keep watching different laws. That can only be archived by having extensive "terms of use". There they set their own, and your rights, what you can or can not do on their service. To maintain any kind of social network, is nearly the hardest thing to do, because you deal with a lot of personal rights. To do so, you have to give the permission for the platform, to do things with your stuff.
Let's say you post a helpful tip on Twitter. Now this tip is on the Twitter-feed. Hundreds of tools sucks your tip with the Twitter-API into websites, apps and who knows where else. Thousand, maybe millions of people can see it on Twitter, retweet it, or share it with other social networks, like Buzz. E.g. Google Buzz even has a special function in it to automatically publish your tweets on Buzz. Same with Facebook and many other services.
Now think about it. If you wouldn't have Twitter permitted to do whatever they want with it, how could they prevent from being sued? Maybe you don't want your tweet at Facebook, maybe you don't want to be retweeted by user xyz, or you don't want to see your tweet on newsfeed. It wouldn't work.
What about media?
Okay, now it's getting tricky. Twitter does not store your picture by itself. It shows it on the new Twitter inline, but you need to host it somewhere else. Flickr, Picasa, Twitpic or your own blog are the usual places. Again, to show, or distribute a link to it, Twitter needs the right to do so. You grant them this right by posting your stuff to Twitter. But how about if you share or link to media from someone else. I think we have fun with this in the future :) But let us assume you only post own content.
Creative commons license to the rescue.
There might be a way to make sure your content is not used by other parties without giving you credit or apply certain requirements. If you chose a creative commons license for your media, which again, is not hosted at Twitter, you can set that others can use it only under the same license (SA), or non-commercial (NC). Also, if you don't want your content to be edited or used for other works, you can restrict this too (ND). This license is proved to be effective in many countries.
Now keep in mind that these may not apply to content you really host at Twitter. That means the written text. However, I did apply a creative commons graphic and note on my Twitter-feed. This whole procedure is theoretical as long as no court has proven it. But I think it's a good chance. Sublicensing is not restricted, the content is not hosted on Twitter, therefore it's merely a link you post and Twitter sucks the content from the link and show it. So you didn't post the content, but only a link to it. So if you give a creative commons license to your media where you host it, the license should be intact. Both Flickr and Picasa have easy functions to apply CC-Licenses to your stuff automatically.
My Conclusion.
I don't see any kind of evil in that. Twitter and Facebook need to protect themselves from getting sued all over the planet. In addition, there is no way to rob your rights. Anyone can write what they want into their terms. If they don't apply to the laws, they are invalid.
I don't see any chance for a service to license content at a site you link to. That would be insane, because thousands of blogs would now be under the Twitter license :). As long as you do not upload your media to Twitter, which you can't do even if you wanted to, the creative commons license will stay valid in my opinion.
Update: Gautam John found a link, which does a good job to explain that content is not equal links to content. Basically the same as my statement.
