Why do they detach the lower part of the cars' rear seats at the junkyard?

I'm just wondering, why do they take off the lower seat cushion of every car before they put it out on the yard for people to pull parts? I don't know if all yards do it, but the two large pull-it-yourself places that I go to here in GA both do. It's the funniest thing, they put a row of cars out, and they are almost untouched--the keys will be in the ignition, personal belongings and trash lying around... except the lower rear seat cushion is invariably no longer bolted down. Why is this? Are they looking for contraband or something? Just curious.