I can confirm that. I made https://pastery.net, a pastebin (THE BEST ONE), and I looked a few weeks later to find that it had a few hundred thousand pastes. I was very happy to see that people were using it that much, until I realized that most of it was spam.
I wrote some heuristics to delete spam pastes, and only about 1,000 pastes were left, about half of which were spam still :(
I don't even know why they do this. They just do it.
Require some complex js codes workflow to post using websocket or something and most automated spam goes away. They are mass mindless bot, not targeted spammers.
That's a good idea, but most of the functionality of the site comes from its API. Then again, it's easy to require a key and a captcha to sign up, so your idea is good, thank you.
Cheaper for their scripts to indiscriminately spam than them manually work out which forms and public sites are indexed and will actually bring them value.
Some will just scribble whatever, where-ever possible.