I think it is pure nosiness. Checking, surreptitiously, is not on. If they have a reason to look into a passenger's belongings they should let the passenger know and they may even be helped. But once a bag has managed to get on a plane, it's generally game over save for "monster bags" which can't possibly be accommodated in the cabin. Nosiness, disguised as security/safety, is totally non-productive and will often achieve complete opposite effect to the one desired.
PM