That is their right, to make the service (not necessarily a website) limit the amount of content, require or not require registrations, have guidelines or not,

That is their right, to make the service (not necessarily a website) limit the amount of content, require or not require registrations, have guidelines or not, to determine who gets to post. But if you do it badly, it can get obstructive. If you have many different ones then you can see what is who like to post on what, I suppose.