It's probably just a business decision. (assumption)They can provide cell phone service for 1000 people for the same cost as Mitnick since he is a target.
It's the same thing Sprint did a couple years ago when they dumped people that called customer service too much.
I'm sure it is, but it doesn't seem like a bright business decision. He claims he spends up to $20K a year - sure, maybe this still isn't worth it to AT&T. But more importantly, you'd think they would see this as an opportunity to make their system more robust for all their clients, save money that way (more than $20K/year? likely), AND turn it into a good PR piece.
It's the same thing Sprint did a couple years ago when they dumped people that called customer service too much.