Those rules are based on laws, which are based on the idea that even a non-blood related legal guardian is in a place of power over a child, and regardless of how the relationship starts, starting one is an abuse of that power.
There are certainly times and places where children who are adopted would need this kind of protection. Whether or not it should matter in fantasy fiction is another matter entirely, but Patreon is worried about losing the institutions that let it get paid (banks, credit card companies, etc).
Do I think writers should be able to write about that stuff without having to worry about that stuff? Sure I do. However, I also think Patreon protecting its interests also makes sense.