I was honestly surprised at how quickly The Avengers found its way to Netflix streaming. I figured there was still enough money to be made off iTunes/Amazon/etc rentals and purchases to preclude giving it away that cheaply.
Meanwhile, Disney can take away a purchased movie from Amazon because the parents probably (a) already have the Disney channel, and (b) are willing to pay for it. https://news.ycombinator.com/item?id=6911944
Do you want to see a real super hero film? Go watch Nolan's Batman series.
The Avengers is Joss Whedon. Modern, progressive filmmakers are pretty good about getting their content onto streaming services, but there are still enormous gaps in the catalogs.
I don’t have the time to search for the rest of the 70 titles in the iTunes Store, but from a few blind tests I did, I’d say the vast majority is available there.