So I'm not one of those people but I think I get where they're coming from.
The issue is one of quantity vs quality. Many of the most prolific OnlyFans posters make the same generic content and they post that content in every community they posibly can to hit the widest audience. It was really bad on reddit where they would post the same picture of their boobs in 15 different subreddits. So far that hasn't been as much of an issue on Lemmy but I think people are woried that if it isn't stopped early then it will start to be an issue and that is why they are targeting all OF posters. I get why OnlyFans posters do that though. OnlyFans is very competitive and very few people actually make any decent money off of it so if you want to make money on there then you need to market yourself agressively and you need to make your content generic enough to apeal to the widest audience.
On the other hand, if somebody is just posting their amateur content without any monetary goal then they don't need to be concerned with marketability. Actual amateurs are free to be much more varied and to only post their content in communities where it actually applies. Sure, good verified amateur content is significantly more rare than professional OF content, but without all the professional OF spam it would be easier to find on here. A lot of people also don't seem to understand the difference between someone who posts on OF as their job and someone who posts on OF because they were posting amateur content anyways so why not post on OF too and maybe make some extra money.
There's also the fact that, like you said, people just don't like being advertized to. That's also probably the root of a lot of the hate.
The thing is that those people aren't selectively targeting the "bad" OnlyFans users. They aren't going through and reviewing peoples profiles in detail. They just see onlyfans and assume the worst.