It's not about safety.
The FAA didn't make this rule. The FCC did, because the cell networks weren't designed for thousands of airborne handsets moving at 500 knots. "Cell phone systems depend on frequency reuse, which allows for a dramatic increase in the number of customers that can be served within a geographic area on a limited amount of radio spectrum, and operating a phone at an altitude may violate the fundamental assumptions that allow channel reuse to work."