Some of these travellers have pre-planned their outburst and rant if stopped. Some might practice 'the look' to not be stopped.
I have also seen a queue being checked for over sized hand luggage and yet - some were called out and told to pay and check it in - others were not. That's one queue for one flight from a small regional field that had plenty of time to check everyone and gain the approval of FR for doing so. But they didn't.