well partially right, seems to be that can have if it's not main focus. to what extent that applies no idea.
Patreon does not allow incest in any size, shape or form. Developers get around this restriction by providing patches. But they can't do it on Patreon, so they provide them here. They can also provide the game in its original incest-themed form on places like SubsribeStar, which do not have the restrictions that Patreon does.
Why does Patreon let them get them away with patching incest back into the game? Because these games are public. Anyone can write a patch or mod for them. The developer can not be held responsible for something someone else does and provides somewhere other than Patreon, like here. And even if the developer is providing the patch himself/herself, that would be difficult to prove.
Other developers, rather than providing a patch, allow the player to provide relationship information at the start of their game, with the default being a not-incest choice. They can then say they are not responsible for what a player does with that choice.
So why do developers put up with these Patreon restrictions? Revenue. Patreon is a long-established host for games of all kinds. So if a developer wants to make money, Patreon is the place to start.