That's a good question. When MS became the 1st console to stream Netflix they wrote the app themselves. They even paid for exclusivity to get the app. So that's probably why they put it behind the paywall: to offset costs of bringing it to their platform. With the exclusivity gone you have point, why are they still requiring Gold? Might be like you said, because they can. Lot easier to keep it behind a paywall when your subscribers are already used to it. But I agree they are no longer in a position to justify it being behind a paywall anymore.
Anyway, back to the app. It was based on Silverlight initially. Since then it has been rewritten by Netflix themselves a couple of times. Once a HTML5/Webkit app & most recently using their own native platform. Supposedly it makes it easier to update Netflix across multiple platforms. As Shortbread alludes to below, we have no idea if MS is being charged any kind license fee to bring it to their platform. I don't think they are myself. Would be kind of backward if they were no?
As I said above I don't think so, but does Netflix make enough money from subscribers to offset their IT & movie/TV license costs? If not, it might make sense for them to charge manufacturers a license fee. But that's just Netflix. No way every media service on Xbox Live requires a license fee too.
Tommy McClain