From the earliest days of websites allowing text input from users, this has been a thing. The same with any website that allows you to draw things, it will quickly show people drawing crude (in content not skill) objects. This is just human nature. Once people realized you could use these forms as attack vectors, we were off to the races. Now that bots can do it for you, I'd only imagine the time before first bot using the form is in minutes. I have seen all sorts of things suggested as workarounds to mitigate bot form submissions, but eventually, you will be spammed at the least with the feature. Adding something like a bot chat, of course people are going to be abusive to it much more than a simple form field.

For someone to be surprised by this today suggests to me that the person is really really new to managing a website.

Exactly. I remember a website from the late 90s that let anonymous users control an LED sign in some shopping mall in Japan. More often than not there were things like swastikas and penises on the board. This type of behavior is nothing new, unfortunately.