I agree, how would they have responded when they find out that the stolen gun was used to kill someone!
It sounds like he was stopped before he exited the store, he witnessed the transaction and possibly saw that the firearm was unloaded before he decided to take action. He may have figured it was an acceptable risk to stop him since the gun was unloaded.
Of course all of this is just a guess on my part since I was not there.
I am glad they re-hired him and hope he doesn't have any issues with them later on down the road.