More often than not, there isn't much they can do about it.
They're getting new fish in at least once a week. Even if they drop those fish straight in to a copper bath, ich isn't killed by medicine while it's on the fish - and a great many of those fish will be purchased within a day or three, often on the first day, by folks who come in on "fish day" to see what's new. It would take at least a couple of weeks to eliminate ich by any method - and your LFS can't afford to hold them, heat them, clean them, and medicate them for that long - and still put them on sale at a competitive price.
I consider it part of our role as aquarists to 1) purchase responsibly, and 2) quarantine our new arrivals to protect both them and our existing charges.
~Bruce