Liability issues. If they'd had a blowout and received major injuries they'd claim to have bought the tire. Lawyers would ooze in and pretty soon the LBS would be out of business. It's why food places don't want people in their dumpsters.

If we got stuff back that was replaced under warranty we had to destroy it before throwing it out. Otherwise, somebody would get it from the dumpster then try to score cash by returning it.