Harry,
Comparing rucksacks to drinking coffee in public are two complete separate issues. I assure you the public does not care if I walk to the gate with a cup of coffee in hand or not and most wouldn't even care if I had a backpack on. I wear my uniform, hat and even throw a bit of shine on the old boots but I will not be told when and where I can have a cup of coffee because some meager and outdated manager thinks he is correct and has a power trip.