Many LFS keep their fish at low dose copper which is enough to suppress ich and velvet. This keeps the fish appearing relatively healthy. However, these doses aren't therapeutic enough to kill the parasites completely, resulting in the reemergence of ich and/or velvet when they're removed from those low dose copper systems. Therefore, it's a false sense of security to assume that they've been treated - even the LFS selling you the fish won't make that claim.
Why do they not treat with full dose copper? Because copper is toxic for both fish and parasites. It just happens to be more toxic for the parasites so it kills them before it kills the fish. However, it suppresses appetite as well as the immune system of fish, which may make them more susceptible to other infections as well (including bacterial). Ultimately, copper for an extended period of time (and at higher doses) results in a shorter life span for the fish.
I know Sea Level doses copper but Warren uses ionic copper, so the levels are different than with chelated copper. I'm pretty sure Absolutely Fish has copper in their systems as well. I know Salgado Aquatics actually does dose therapeutic chelated copper at levels above 2.0. This is probably the reason that all of their fish eventually get discounted if they've been around for a few weeks - they gotta move them before something happens to the fish.
You can check out this brief post by Jay Hemdal:
Trying to understand masking ich with copper
Hi all, first just wanted to thank you all on this wonderful forum for the advice and knowledge so far, my first attempt at quarantining is going very well even with a tank full of ich :) So I had a thought regarding ich and copper. I understand that copper is ineffective until a minimum...www.reef2reef.com
Thanks man