That's generally because plastics aren't made with the same level of food-safeness. So plastics made to be used with foods are generally more heat-safe and stable (so they won't leach out the ingredients in the plastic as easily). Even so, the most concern would be using non-food plastics with wet foods or foods that have acidic ingredients in them. Unless popcorn had a coating of butter applied after baking, it wouldn't really be wet especially after it cooled off, and popcorn doesn't have acids. So you could probably use it.
But, better to put a sheet of plastic wrap or aluminum foil or even fabric inside the bowl to separate it from the plastic, or another bowl of any kind (metal, glass, ceramic, paper, etc).