I believe Paypal is Patreon's largest single payment processor and as such, they can institute certain rules over what is and isn't allowed.
Paypal is notorious for their attitude towards adult content as, primarily, it has much higher than average fraud rate. This is especially prevalent over borderline illegal content (barely legal or legal by a loophole). Additionally, adult media has a much higher rate of return compared to other media, dissatisfaction and requests for reimbursement.
Note that even CCBill, which was set up specifically for online adult content, strictly regulates websites that use its services.
I personally know two examples, stated in support tickets I requested.
One was Karup's PC which was forced to retire a lot of content posted before 2009 which didn't have complete 18 USC 2257 paperwork (Karup media complied, while ATK either had complete paperwork or they weaseled out of it for their early content).
Another was Met-art. Other than obviously illegal content from unscrupulous Russian photographers submitting underage model pictures, they were also forced to take down model pages for some models who looked too young, e.g., Josephine photographed by Peter Dominic -- even though she was 21 when photographed and even though she was their public face on the cover of two issues of their web magazine (with an interview and a cover story).
Patreon polices adult media creators with a lot of scrutiny because on the one hand, non-adult creators generate the steady bulk of their revenue and on the other hand, if they overstep their bounds, Patreon stands to lose Paypal as a payment processor. If that were to happen, Patreon would ban all adult media from their site with virtually no grace period and no migration process.