They aren't charging you (well, directly), they're charging the device manufacturer -- the $1/Roku amount is not the actual per-device royalty.
Good thing as a customer I don't care. I can't imagine Roku would sell their device for $98 without Amazon, nor would a $69 device without Amazon Instant Video seem attractive.
As the customer, I couldn't care less. There is no "lock in" in this regard.
That's still much less locked in than Apple's approach (IIRC another manufacturer asked Apple to name their price, and was told they just weren't interested)