You can argue it's abuse or illegal or fraud or whatever you want, but here's the thing: how are you going to stop them? Sure, maybe you stop Facebook with a lawsuit... but everyone else is still doing it, even people outside of your legal jurisdiction. They're still going to do it, so it's up to you to stop them with your design. If someone breaks into your website and destroys a user's data or steals their credit card, that user is not going to want to hear "but what they did was against our ToS!"

This isn't your house where there are police patrolling and ready to respond at a moment's notice when they're called. This is the Internet, accessible by almost literally everyone on the planet, and they don't give a shit about your policy. That's why best practices and application security was invented. So use it.

"Hello, I am a HTTP client, can I have /some/super/secret/page?" "200 OK, here it is"

That's your server complying with the request. Whether by intent or by oversight, doesn't matter: the client comes and asks, and your server can refuse. If it complies, well, you told it to. Whether you have merely exposed the page to the public or also shouted its URL from the rooftops, that's completely irrelevant. If it's not supposed to be public, don't make it public.

"Hello, I am a HTTP client, can I have /some/super/secret/page?" "Oh, but you are ^User-agent$=.acebook ? Nope, 403 Forbidden, no data for you." (Or, more generally, "And who are you? 401 Authorize!" - or any other sort of mandatory access control)

Someone viewing a webpage you put online is not at all like someone stealing something you own.