It would be pretty simple to open a web page, do a few user interactions that shouldn't trigger this behavior, and then penalize or deindex web sites that hijac

It would be pretty simple to open a web page, do a few user interactions that shouldn't trigger this behavior, and then penalize or deindex web sites that hijack the back button. Add manual penalties for the ones that slip through the automated testing. Once it becomes known that doing this means saying bye-bye to your traffic, sites will stop doing it.