I cannot tell if you AMP will shake out to have been a good or bad idea for the web.
I can attempt to answer this question though: those sites don't care where you read them. They just want people to read them. Read them. Like them. Trust them. So when they have something to sell (in any sense of the word), they sell it. Losing that attention is more scary than losing direct traffic to one place it publishes. Without saying it in so many words, AMP is saying: you stand to lose that attention without this.
And of course it's not just AMP, there are loads of off-site places you can publish to. So many that I think publishers are saying: "whatever, just make it easy, and we'll blast our content wherever you want."