Justin,
Maybe if I were to see a single person ask a gunshop employee if he could dry fire a weapon before picking it up off the rack and doing so, or if I'd been chastised in the past over it for anything except the one time I dry fired a .22 ("don't dry fire .22s, it's bad for the pins" - no mention of centerfires). Then I'd start to entertain the possibility that it was common practice.
As for the good mannered part, I can accept that it's good manners to ask before doing something. However, given what I've seen of the behavior of both gun shop owners and gun shop patrons, I'm not going to assume it's rude to dry fire without first asking unless there's a sign saying otherwise, particularly with used guns.
Again, this guy didn't say "ask before dry firing" he said "don't dry fire in gun shops". Big difference, and the whole point.
Anyway, the broader question (or statement) I was trying to ask/make was along the lines of, "No wonder people don't like to go into gun shops. The majority of those in there are ignorant, pretentious people and will bark at and/or chastise a paying customer* harshly for a perceived fault." Kind of a, "hey, if you own a gun shop, don't do this - it will drive off customers" warning.
*Ive bought a number of things from this specific individual in the past.