Now seriously, isn't it obvious? It's a high-security area. There's all sorts of people trying to cross the border: business travelers, families, citizens returning home, tourists, illegal immigrants, terrorists, drug dealers. They don't want people distracted, speaking loudly on their phones, or sending instructions to others in line to exploit security weaknesses.
If you're really bothered by the wait (and have an extra $85 to spare), you should seriously consider requesting a Global Entry. While you still won't be able to use your phone, it'll reduce your time in line to 45 seconds.
I bet you can't even finish a level that fast.
(UPDATE: cost for 5 years is $85, not $100)