The fact that even with the Black Card (any location) membership, you still have to be tied to one "home" location and can only manage your plan at that one location is also predatory. I've read stories of people calling into Planet Fitness corporate and eventually getting a customer service rep to cancel their plan (when the location refused to do so remotely), so it's not a limitation of their system and it's not a legal restriction, it's just another way they make it difficult to cancel.
I will mention, one loophole for at least getting around a bad Planet Fitness location (e.g. a manager pretending they're not receiving the cancellation form in the mail) is going to another location, having them transfer your membership there, and then cancelling with them. I've done the store-and-back thing for changing plans before, and the managers oftentimes don't care/are happy to help with it.