The 360 is the super otaku/pervert/nerd machine in Japan. However this happened (easy to license/develop for, most likely) the thing is that once this starts to happen (years ago now) anyone making a shooter or a galget game is logically going to put it on the system with all the other games of its kind since those sorts of people tend to buy a lot of the same stuff. I'm not sure MS had all that much to do with this. Maybe it did in the beginning, courting smaller Japanese developers since the bigger ones kind of gave them the bird.
I don't get the title of this thread, BTW. I understand the "SHMUP" thing, obviously the result of a brain injury, but what is meant by "another Treasure SHMUP"? Is there, like, this huge pile of Treasure shooters than are accumulating over on the 360? I can only think of two or three ever, depending on your definition of "shooter". I suppose you could include Bangai O, which was also ported to DS, and Sin and Punishment, which is on Wii, along with its sequel. The pile of Cave shooters is much bigger. Bigger than what is even worth owning, IMO.