Thanks for the info Michael.
I would just like to add that this kind of requirement isn't so much a predatory action as it is a self defense mechanism. Lawyers of any company with enough revenue to make them a lawsuit target go over contests like these with a fine toothed comb to make sure there's absolutely no way for the company to get burned. In the case where you are submitting a game concept or prototype to a retail publisher or a contest run by a company that also makes games, there's a very real risk that one of the concepts submitted may resemble something already in development. In this case (regardless of how well it would stand up in court, when their existing title is released the developer who submitted their demo could file suit on the complaint that Microsoft, or the publisher "stole" their IP and used it for their own gain.
Then there's a huge legal hassle where the publisher has to prove otherwise etc. yada yada yada, in the end they just take the stance "we're trying to offer you a service if your not prepared to work with us then don't bother apply". The "We have rights to anything you submit" is just a way of covering their @$$'es and removing one avenue of litigation. Regardless of how people on the XNA team may feel, they probably don't care at all about having rights to yor IP, Legal wouldn't let a contest like this off the ground without that clause.
Those with experience in the industry should be fairly familiar with this scenario and to be honest comfortable with it. Any game developer worth his salt should have more then 1 idea anyway, if you don't want to waste your precious on a contest then don't. Use your game developer skills to come up with something you don't mind submitting to the contest. It's really not a big deal