I think KH3 will be PS3/4 only too, but I don't think that means this collection is PS3 only for that reason. Even if Sony has locked down KH3, they'd never pay to lock down the HD Collection, so it'd be basically free money to put it on 360... If SE were willing to throw the resources in it, which I think is the real reason. The team for the port was very very small, Sony provide development software for HD Remasters already, and the 360 and Wii U market in Japan is worthless.
SE published the remakes of Star Ocean 1 and 2 on PSP, and they included a trailer for SC4, which was initially 360 exclusive in the ports. I don't think they care even nearly as much as this concept suggests.