"Our license to include the YouTube app in iOS has ended, customers can use YouTube in the Safari browser and Google is working on a new YouTube app to be on the App Store."
Translation: "We had paid for a license to include YouTube in iOS until now, but now that the license has expired, and we have significant market power, we don't want to pay any longer. We know Google will create a stand-alone app, so why pay them?"
Until Google releases a new YouTube app in the App Store, parents whose young children have learned to use the YouTube app should forego upgrading their devices to iOS 6, unless their children are just as good at navigating in Safari.
While iOS 6 devices will lose the built-in YouTube app, iOS 5 devices will not. It is unclear if it is because Apple's prior licensing agreement is worded such that YouTube can remain on iOS 5 and earlier, or because Apple didn't want to push out a point release to remove YouTube (or both).
Admittedly, since we're talking business here, and since Apple has gone thermonuclear on some Android OEMs, we'd guess the first of the above options.