That's going to be a problem for people who didn't play the originals anyways, doesn't matter what system they happen to buy KH3 for. I've never played any KH games, yet having the game be a Sony exclusive doesn't mean that I'm any more likely to play it.

If someone buys the third game in a series expecting to understand every single thing that happens, they're in for a disappointment regardless of their console choice.
Besides, unless KH1&2 are available for download via PSN, there's no way for anyone to play it without tracking down a PS2 copy.