I think this is the best fitting analogy so far... You can extend it to further talk about content owners, publishers, user-generated content, paid subscribers and free readers, etc. Newspapers also deal with existing regulations that address the same type of issues, such as first amendment rights vs. libel laws and the public's right to know vs. need for privacy.

Here's one I just thought of. Maybe not entirely serious:

Prostitution is illegal in most states. SOPA is nearly equivalent to shutting down a bar because somebody went into the bathroom and scrawled "For a good time, call XXX-XXXX" on the wall.