I can think of only one reason to prohibit cell phones in Customs: the same reason they're not allowed at the counter at the Post Office. People who talk on cell phones tend to pay attention to nothing else, and they don't want anyone holding up the line, and they don't want to have to compete with your phone for attention when it's your turn.

Same reason cell phones have to be turned off during taxiing. If a tire catches fire and they have to evacuate the plane, the flight crew don't want to have to rip your phone out of your hand to get your attention.