The most basic function of Customs is to know what is coming into the country. Customs has always had the right to search whatever and whoever they want.
It really says a lot when a gov't is afraid of information.
If they're searching information, they really need to be putting multi-minute delays on all cross-border internet communications while they search the content.
More like multi hour if you want to block from tarmac to past-customs. It takes at least an hour to clear customs in SFO (For a non American)
the cellphone comes into the country through the airport, but the information on the cellphone has no such physical barriers.