Well, they do actually. They have a lot of restrictions on what 18+ content you can publish.
Like you can't publish "real" porn, or that's how I interpret this at least:
> Works depicting sexual activity between real, adult participants are not permitted.
Someone producing adult content within the boundaries of what the guidelines allow did pick the right provider, so I don't see a problem with them becoming a prominent creator on the platform.
I should have probably said I don't expect or want them to micromanage the prominence of creators on their platform that do act in accordance with their guidelines. If for some reason figurines became incredibly popular and someone would complain that Patreon is basically just a platform for figurine painters, so what?