Is it because they feel there's characters in it who look younger than 18, regardless of being told they're 18?
About the coerciveness of the MC, manipulation of young girls in the story?
It isn't about Patreon, it's the money processors they're reliant on.
They tend to be run by religious fundamentalist groups who have lots of money and power.
More importantly.
It's not out of protecting anyone, especially not kids.
It's about keeping the discussions about those topics out of the mainstream.
Netflix and Prime will all feature films and shows which would never be allowed by Patreon.