So, how *can* you do this? Can't they sue you?

Anybody CAN sue almost anybody over almost anything. That doesn't mean it's usually advisable, however.

There is a wonderful little federal law known as Section 230 of Title 47 of the United States Code (47 USC § 230).

Summation:
c, 1 "(1) Treatment of publisher or speaker

No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider."

For more information on what that means, read here:
http://w2.eff.org/bloggers/lg/faq-230.php