Unlike UK airports which tend to be on two levels, one for arrivals and one for departures, US airports have always mixed their passengers. They have always screened anyone entering this 'sterile' area as Matt says so you don't pass stuff on to departing passengers. I assume the don't trust the checks at the departing airport. This is also why the duty free is taken to the plane door, so you can't pass it on. Before 9/11 you could go to the gate to wave friends off as well.