We already had users posting pornographic pictures, or stuff like that. I think that's a good enough reason.
At the very least require someone to add something of possible value to the site before adding something very terrible.
The Math Help Center's Privileges page gives you a full list. If you're logged in, the dialog also shows the ones you've already earned with a green checkmark on the left side.
One way you can get to this dialog is by clicking the Help button, which is the one with the question mark second from the right side in the buttons list at the top right, as shown ...