Probably to avoid getting sued. Easier to say nothing than to say something stupid.
Discussion
They should have a public statement they point to when they can't give detailed explanations, like the source of the law. Or is that secret too?
Again, easier to say nothing than to possibly risk something. If you have legit reasons for blocking someone, telling them may allow others to bypass your legit reasons for blocking.
Not defending, but thinking like a company might