Most LFS don't run Cu at a level high enough for a 100% ich kill. Diver's Den, perhaps the best source of fish on the planet, runs Cu 24/7. they strongly recommend that you use a QT with their fish. If a lfs runs copper and has flame angels in the system (as DD often does); you can bet that Cu isn't at a level high enough to kill ich. The FAs are very Cu sensitive. Cu also kills algae and countless other organisms; a big reason for dealers to use it.
Fish often go through some LFSs so fast that buried ich, which is not killed by CU, will still be there when you buy the fish. If ich shows up in your QT, its fairly easy to treat. Velvet or brooklynella, both can slip through the light copper defense, are far more dangerous.