many airports have signs banning the use of cell phones in 'senstivie' areas which include both immigration and customs.
There are many reasons for this, but all are essentially for law enforcement purposes.
As a law enforcement officer you don't want a potential suspect tipping off that they have been caught, or that someone else has been caught doing an illegal activity. You also don't want cameras (on phone that are equipped with such) to take photos of sensitive information which may appear on computer screens. These can include crimial files which come up on early warning systems.
Countries banning cell phone use in the arrivals and customs hall include Australia, China, the UK and the USA just off the top of my head...