Some of the testing they can do, the signal strength is a little tricky, which is why we came up with the TIU signal tester.
However, many hobby shops don't have things like oscilloscopes and the like, so they might think testing the TIU with a locomotive on a test track is good enough, and it's really not.
Here's the whole thread on the TIU Tester project starting with Adrian's original idea for it.
Design of a $10-20 DCS-TIU Port Tester Tool?
Here's a quick demo I did with the TIU signal tester.
A shot of the actual PCB.