Unless you have an ironclad way to verify the identity of every poster, you'll get a bunch of illegal stuff anyway. And if you're a "utility", then who is responsible for removing that from your service?
What you're really suggesting is "common carrier", but the difference is most common carriers carry the content in a point to point way, not publicly (FedEx, UPS, your telephone provider). For the common carriers that do have public broadcast (radio, TV, cable) they either have to moderate all their content and are responsible for it (TV, radio) or they have to provide an unmoderated but public access section where they know exactly who the content provider is (cable TV, who gets to choose which channels to carry as long as they have a public access channel).