When I bought my 90 300 SEL from a private individual a couple of years ago, I went to the dealer for a few parts and for a small service item (don't even remember what it was). I asked for the records and was told they "weren't allowed to do that." So I asked for the service manager (big, glass enclosed office). He informed me that it was "MB policy not to provide service history except for work performed by the current owner." I'm not a hothead, but expressed my dissatisfaction with that 'policy' in a respectful way.
A couple of months later, when I had something else done by the service department, I asked the (different) service rep if he wouldn't mind running a printout of the history. He said, "Sure!" and handed it to me without a thought.