If Nintendo can get away with putting a SNES Rom on a DVD disc, I'm sure SEGA can be forgiven for picking up 10-12 year old games from the Dreamcast library.
Though, I do see your point. All of the Dreamcast games have aged greatly, even back when they were compared to their Gamecube and Xbox counterparts, so expect some nasty critiques against an HD version of Shenmue.
It's a rough period, The Dreamcast got into the game, pretty much when the Playstation I was in it's last legs and the Nintendo 64 still had some juice left. But even by early 2000, the Dreamcast was far superior graphically. Of course, in 2012, a lot of DC games look ancient. But a few of 'em are still kinda presentable.