As to sliding scale morality... some would say if you are going to drink beer you might as well steal it and kill the guy you stole it from. So, whose setting the moral standard is a good starting question. At work when I get asked if I dont have something better to do, im frequently tempted to say "Thats what the guy who I caught spitting on the sidewalk said, so I kicked him lose and found you."
Having said that, I would not keep a commercial keg even if I found it or someone tried to give it to me. It just isn't worth losing my ability to say I have earned everything I have. That might be higher standard BS but thats just how I am.
Regardless, not returning a keg is a civil issue, not a crime.
Edit: A cool idea comes to mind for a retirement gig. Keg Repo Man. Maybe parlay it into an A&E show....