If I buy or rent space for a billboard I am not allowed to put anything I wish on it. That is no different than putting a poster in a public-facing window. It gets more interesting when a website in one country is offensive to another country.

I'm pretty sure that if you put a two-story tall porn image on your property it's not going to last long. I'm pretty sure that if you post a hugely racist message as big, or some other things, not matter how public that space will be, you'll run into some problem with the law whether at local or federal level.

Whether you're a private or public platform here is not as relevant as the fact that is is a public forum, it is practicing certain forms of censorship, and it a medium of information. Other platforms get regulations applied to them in many respects too.

Governments do in fact regulate posting of notices, including both commercial and political speech, routinely. This includes both forbidding specific types of notices in some instances, and compelling them in others.

As only one of literally millions of examples: http://ij.org/case/gilliland-et-al-v-city-of-dallas/