When you consider the vast amount of words, pictures, and videos that are posted constantly, to check everything for 'bad' content would take a huge army of people working 24/7.
And then of course, people don't realise that checking everything means some stranger going through all their posts and making a decision as to whether to allow it or delete it.
And with these sites reaching so many jurisdictions, whose model of decency and legality do you use to decide what should be censored?
And finally we have the looming issue of 'Truth'. Who decides it? Truth can be a slippery thing in some cases. It is not always easy to ferret it out. Hell, if there was a law that journals could only publish established truth, every newspaper in every country would have to shut down immediately.
There's a solid argument past the flamebait though. When your company becomes the de facto public commons, a forum, and isn't, de facto, just a private brick and mortar shop(the usual concept of a private space that you can arbitrarily ban people from) anymore, what is the appropriate response? That requires some education in history, political science, philosophy, etc.