In addition to the UMS stuff Greyface mentioned, I feel like it's also an incentive to get people to use their services rather than dump everything on their phone, though the Nexus 7 is more guilty of this than the phones.
How does the Galaxy S3 handle the flash+a memory card? Does it support UMS or is it also MTP only?
From what I remember I think it was MTP only out of the box. You got to choose which storage to browse when you opened the GSIII in Windows Explorer.