If that's the case, then why have a regulation against the usage of cell phone all together if it doesn't even work when you're off the ground? I can understand the regulation if it actually does work at that altitude but it doesn't so why have the regulation at all?
Because no matter whether there are cell towers nearby (and at that altitude, obviously there aren't), your phone will try to communicate with them.
And because it won't get any answer from a cell tower, your phone will give maximum power to its antenna for communication in hopes of getting a connection and that's what makes it a problem.
It's not as much about having an established connection between your phone and a cell tower as it's about your phone transmitting as hard as it can in order to get one.