Prime Video has been struggling so Amazon can't afford the strategy tax of limiting it to their own devices [1]. The more Apple TVs that are sold by Apple, the more untenable their position becomes, especially when Netflix will be on any device. Amazon vs Google might be turning into a hot war (which makes sense since they're competing in an increasing number of businesses), but that's not the case with Apple.

[1] https://www.wsj.com/articles/where-amazon-is-failing-to-domi...