I'm the Loss Prevention Officer at a retail store, and we do NOT check customer's bags or hold it for them because of the fact that it comes off as rude and offensive as OP is taking it. However, I will be watching that customer for the remainder of his visit because well, that's my job. As long as the backpack stays on his/her back there's no problem and I never accuse them of anything other than being regular customers, unless I have enough reason/evidence to prove otherwise.