First, from a legal standpoint, "just use Android" isn't going to fly. Like I said, what is interesting about this case (for me) is that this isn't cut and dry. Apple has basically said that nobody is free to sell software for iOS outside of their own store. Is this illegal? We shall see!
I am not sure where you were going with the "Apple products on Android phones" comment. Apple is already free to sell their Apps on Android if they want. They are also free to set up their own Android store if they want. Of course, we will see pigs fly before this happens, but you get my point.
Apple likely won't be able to ask for a cut of the profits in third party stores if Apple's SDK isn't use to produce said app. The same stands true for any other operating system, I believe. Just think in terms of real world tangible goods. I create a product, let's say a vehicle. Somebody else makes some really spiffy seats for my vehicle. If they are not using parts that I patented, they can't (to my knowledge) be required to pay any sort of royalties to me, the car manufacturer.
And to answer your last question, if you have a nook, you CAN buy books on the kindle store, yes. The more important thing to note though, is that I can buy an epub from any store anywhere and use it on my ereader, be it a nook, kindle, or whatever else. I don't have to have Amazon's or Barnes & Noble's blessing.
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I am pretty sure you are spot on here (regarding many responses to this thread).