Think of it as whitelisting vs blacklisting. In the US we don't whitelist the things that are OK, we blacklist the things which are not OK. How did cell phone use in a border area get on the blacklist? If it's on the blacklist, but there's no law which put it there, then there's a problem.
The thing you have to understand about the border control area at airports is that it's kind of a "no man's land." Despite technically being on US soil, you aren't considered inside the borders of the US. You go inside only when the immigration officer gives you the green light. Until then, they can basically do anything they want to you, and there isn't anything you can do about it.
Otherwise if it's truly a no-man's land I could take a plane trip with another person who I intend to kill, depart the plane and while still in the border control area kill him in cold blood in full sight of everyone there. The US might want to prosecute me, but they technically can't because the law doesn't apply there.
I don't think that my murder example would fly. Do you?
Hahaha, what? You can't be serious.
I lost count of how many times I had to (respectfully) correct border patrol officials when stamping I-94 forms, or filling the number of travel companions in the customs form, etc.
Thankfully I'm one of the good guys.
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)