Let's be clear here - OF creators are looking for unsolicited free advertising.
And that's at the crux of the issue - both free online communities and subscription based paid online communities are competitors because both are in the business of content generation.
Free online communities make money from advertisements. So it is in their obvious interest to discourage all attempts, by anyone, of "free" advertisement. Especially their competitor!
OF creators are in the business of making and selling their content. Obviously it is not in their best interest to make their quality content available freely. As free quality content is anathema to their business, obviously there is absolutely no reason for any community to tolerate their attempts at free self-promotion.
The solution? If you want to make money, you have to spend some money - Just pay for a suitable ad in the free online communities and support them.