I did a quick Google and found this:
http://www.usimmigrationsupport.org/...smuggling.html
It is to prevent arriving passengers from having communications with a corrupt immigration official. I assume this was a method that incoming bad guys used to talk with the corrupt officer to find out what booth he was at, and then somehow go to that booth.
My other guess was that cellphones are prohibited in prisons, and CBP wanted arriving passengers to have the experience that they were prisoners.