Spam can be blocked, though, right? Edit: To address the commenter below, Congress has addressed spam before, in the CAN-SPAM Act of 2003.
Only content that is literally illegal or is spam would be removed.
See where this is going? It is hard to craft rules that allow everything without leaving a site unusable.
Does government cheese have to be tasty? The point is to provide the public square, in all its public square glory. If you want something slick and curated, you're going to have to give up some rights.