I totally agree with you that there is no official rules for any communities and should not be.
However, I believe anyone can see what is relevant as far as they have common sense. You do not need to decide. You actually just know.

Insisting to put #kr tag to any contents that is not relevant to Korea at all is like to me that putting #cooking to contents about cars without relating to food or cooking at all. That is still absolutely free to do but doing that will definitely not be welcomed and have no point excepting for annoying people interested in cooking contents.

I also respect your opinion and hope what I said was well understood.