I've recently looked into having my glass tinted, I'm led to believe that the law states that both front side windows must allow 70% light transmission.
Having spoken to very reputable local company offering this service it appears that the OEM side windows on the 370 are pretty much on the legal limit! This is not uncommon on new cars. I was also informed that my insurance could be invalidated by the application of a tint film, it can actually prevent access through the glass in the event of a RTA.
You are correct. Both 350/370 leave the factory almost bang on the limit.