I don’t think that in every case it is necessary down to user rejections. I can’t believe that in the netflix case users actually prefer to have to login with two post backs, one for the login, one for the password.
It wouldn't surprise me if people with active netflix accounts tried to login into that page which is why they changed how it works, also like with any A/B testing it's not always clear what user behaviour they were seeking to change or reinforce, and it's usually more than one.
For example the buy now vs add to cart only on the Amazon one might have been looking at more than just how many products are sold, they might were also been trying to see if they can say reduce impulse buys that result in returns without lowering purchases that do not, in fact the reason they've kept the buy now might be because it actually reduced the return rate as people interacted less with the site and didn't buy additional items that they returned later.
That's actually not a login but a signup form. More people will probably enter anything at all with only one field showing. And since they've already interacted with the site, they might feel inclined to complete the process even after they find out it's multi-step. Some will of course bounce because of it, but it should be net positive overall.