Xbox 360 = biggest available market. Add bonusses they presumably get from Microsoft and the deal/management decision is perfectly logical. 3 Demos are more work to do than 1, even if you have a multiplatform engine. The servers have to run great aswell, lag could kill the impression people get from the demo.
I'd sure be glad to see a PC demo, preferrably part of a Singleplayer mission.
But first of all I'm not buying any games before or on release anyway. I wait for reviews and more gameplay footage. By the time I buy a game I already know I'm going to like it.
And secondly demos are not even a sure sign of final quality. Couple years back I bought Test Drive Unlimited because the demo was breathtakingly awesome. The final game disappointed me, I didn't feel like witnessing a realistic island with fellow drivers anymore, it was just a half-decent racing MMO. Maybe the aiming or graphical filters be a little different in the final game and 360 users will be the ones complaining while both the other platforms are happy with what they got. Or the PC controls and the graphics on PS3 will svck and the 360 users will be the least disappointed

Who knows at this point - let's just let it play out. Hours ago I was sure there would never be a demo at all, just now I have returned to a place with internet access and watched the, imo, VERY promising trailer. About the Halo comparison: Invisibility and ground punching special attacks are exceptional moments in Halo, here they will be core mechanics. The gameplay will feel totally different as far as I can tell.
Graphics and physics are much more realistic and versatile, the maps offer a nice variety aswell! Everything about this trailer is a positive surprise to me, but I know I have it easy as I own a 360. Didn't get into the Alpha test, was disappointed just like all the people who don't get a demo now. In the end you'll all find out whether the game is what you want to play and either enjoy it or look for something else. I really don't get the riot that's going on here.