On the other hand, do you want to get involved with people who are okay with such gated communities? It sounds to me like there would be a difference in values.
Interacting with people of different values is mostly a good thing. And it is certainly no bad thing when the differences are over something as trivial as which online platforms you want to use.
I think the reason why Facebook is used is due to the ease of setup and management of the group. Rather than installing forum software, paying for hosting, etc., it's just easier to use Facebook Groups. And, as adrianratnapala wrote, I think it's good to interact with those that have different values.
It's not a difference in values, just a difference in priorities. You make it sounds like they explicitly set out to build a gated community, when they actually want to build a community and just choose the path of least resistance.