There is no way to no for sure. But, my guess, at least for the majority of them, is that they are very angry at the people who make such comments and look for such pictures to find such comments to engage in.

For myself one lady I knew posted some pics on social media of her daughter doing some amazing gymnastics stuff. It got a lot of attention pretty quickly and there were a lot of comments ranging from more innocent like "you are so pretty" to plenty that were very sexual toward her.

The mother deleted the pics, vowed to never show her daughter online again, and for a few weeks went finding people making such comments, going to their friends list and linking the comments the person had made to them and saying did you know your friend/family member was a pedo? Or in the case you are talking about above they just flat out call out the girl and her parents for posting it. But, as for my experience. I found very few doing that. It was more people responding to posts by people asking to meet, saying they are sexy, asking them to do sexual things to the camera, etc.

I don't do social media anymore, but for a while the algorithm was very good at showing me pretty young girls. I'd bookmark them for later only to find the account would be wiped clean next time I visited it. I started to only bookmark if the account had been around for a while since 95 times out of 100 the account would be gone very quickly. I imagine that's the cause in most cases.