Why is Sony to blame if the Kinect doesn't take off? At the end of the day, the market has to be there for it and MS and developers have to be serious in investing in it. Regardless of what Sony does (and its well within their right to have taken advantage of the situation), the blame has to be put on MS for their initial muddled message and (for lack of a better way to put it) consumers for not caring.
Because they took the easy road: they had plans to bundle their camera too, realized it sucked in comparison to Kinect, and opted for the cheaper price point just to get sales. Not surprisingly, those higher sales came because consumers are, generally speaking, cheap Luddites. They don't like change (hence, we're still gaming with the couch-and-controller setup of 1979 and DVDs are still more popular than Blu-Ray or digital), and they want things cheap. Yes, this is "the market speaking". But the market is bad for technological progress. It is only when companies force innovations on consumers that progress is usually made. Microsoft was doing that by bundling in the Kinect. Sony got in the way with their cheap strategy, forcing Microsoft to bail on the promise of the Kinect too. That's why Sony is to blame.
That's not to say that all blame is on Sony: a good chunk of it is on Microsoft for having a terribly crafted message pre-launch, exacerbated by some (now former, and rightfully so) moronic executives who acted like arrogant nitwits on Twitter instead of explaining their strategy properly, and by not having enough quality Kinect games at launch (Xbox Fitness is pretty good, but it's pretty niche). Kinect Sports Rivals should've been 1. better, 2. bundled in for free at launch with all Day One editions.