In my experience, unless it is truly a local FISH store and not just a pet store with fish, the LFS employees often know little or nothing about fish and many don't know what qaurantining IS. The last 3 times I've bought or considered buying fish at the only local store I'll shop at I was told that all their fish are healthy and not to bother keeping them QTd at all, just throw them in with one of my prized fish. Yeah, 3 days later major major ich on all of them.
When they get shipments or trades they go right into the system and everything is exposed.
Point is, you can never tell by looking at a fish what it COULD have, only what it isn't showing signs of yet.
I feel if you don't quarantine new arrivals it's your own fault if they catch something. Learned that the hard way on my own despite knowing better.
Plus, they can hide/carry many diseases until the right stress or trigger comes along to ignite it.
99% of the time customers in some LFS don't care even if you warn them they should qaurantine and hey, maybe quit buying fish every other week from walmart since they all croak, but they are going to be back it a week whining that all their fish died mysterious again and buy a bunch more.

It's tiresome and even a pretty good LFS isn't perfect.