You are correct. But if Roku tried to force Netflix to do, well, anything, Netflix could take its ball and play elsewhere. Roku needs Netflix more than Netflix needs Roku.
Of course, it's not that difficult to stop the streaming. Exit the app. Or hit the Home button on the Roku remote.
I dropped Netflix some time back, so I don't experience the issue, but family members certainly do. I think it's a poor user experience. But, for the no attention span group of users (that seems to grow every day) it's not an issue. I'm not Netflix's target audience, so what I think doesn't matter to them. So, I keep my money. Or spend it elsewhere, actually.
Roku Ultra (4660)
Apple TV (5th gen), TiVo
Roku boxes from every generation.
Apple TV (2nd, 3rd, 4th gen)