I can understand making a few modifications but these booths were to promote the XBOX itself. They were not the developers promoting their games. For the developers it is understandable for them to have the a stable system. Microsoft however is trying to pass off a PC as the new XBOX one with nothing in this setup remotely close to whats in an XBOX.
At least Sony put up their dev kits. Hey they crashed sometimes but I can forgive a system thats in beta crashing far more then another company deceiving people into thinking that this new system can do all this cool stuff in the beta version but not be the system.
They can do what they like. The fact of the matter is that unless someone pulls out a 'release ready' Xbox One and demonstrates that it can not perform as expected, failing in such a way as to indicate that using a PC was the only way they could make it look good, it really makes difference.
If it works when it's needed to work, that's all the matters, does not make a bit of difference what they used to make it do so during some demos.