I can't speak for Facebook, and I'm not super with-it on the conflicts in Spain, but it seems like they're making the distinction between "we want our own country" and "we want our own country and we want to violently purge everyone else from it".
I'm reminded of the Sargon v Richard Spencer debate here. Sargon's line of attack was essentially this: You advocate for an ethnostate, people will not react well to the non-violent policies you publicly advocate to bring this about, thus there will be violence, therefore your rhetoric is violent. Even if you approve this line of reasoning, you have to admit that it can be applied to any separatist movement anywhere in the world.
Nor should they. I helped run a large, active web forum for years. We found general rules and moderator discretion were far more valuable than a list of 800 different "you can't do foo" specific rules, because the bad actors tie up all your time lawyering about the specifics.
e: But it's not an arbitrary exercise of power when I do it!