They are not. Or rather they have generally been deemed unenforceable as such (at least in the US). You can, of course, be banned by the service for violating t

They are not. Or rather they have generally been deemed unenforceable as such (at least in the US). You can, of course, be banned by the service for violating them. This is similar to signs regulating behavior that businesses might put up. By and large those are not in any way legally binding. Generally all the proprietor can do is ask you to leave. But since they can and often do ask you to leave, people generally abide by the posted requests. In a similar vein "not liable for THING" signs are more accurately read as "we don't wish to be liable for THING and are going to attempt to refuse if at all able".