Id say tomb raiding is when you're taking something for a better purpose, where as tomb robbing is taking something for selfish reasons.
If you steal an ancient scripture from somewhere so it can be put into a museum: tomb raiding.
If you go to a burial ground and steal all the gold off of the bodies: tomb robbing.
Even then though- id say if you stole all the gold from the bodies so that you could buy a sword to help you on a quest to safe a town infected with orcs for instance, then you're doing it for a greater non-selfish purpose: Id call that raiding
Cool question