Say, the Facebook Events calendar is enabled. I disable it. And after some time I get a reminder of a Facebook event I am not even attending (read: I am not in the list 'going', I am in 'maybe' list, or I saved the event). So, when I click on the reminder, suddenly my Calendar is full of Facebook events I am either just invited, or I am in the 'going' or the 'maybe' list or saved the event. Thus, somehow by clicking on the reminder I enable the Facebook Events Calendar.
If I would just get reminders of events I am attending on Facebook and only those would appear in my Calendar, I would not mind at all. The problem is that I get plenty of invites per day, the one more interesting than the other. I already added certain people to the list of people who cannot send you invites, but the current people do organise some really good events (apart from the other less interesting), and I do not want to miss those.