If Google allowed other App stores on Android then maybe Amazon could make one. Or maybe they could add a setting to allow users to install their own APKs somehow.
It also involves around three or four "I know this could be dangerous" click-throughs. That is harder to get an audience of everyday, than settling for paying someone you are probably already paying.
The average person will not click through a security warning. And if they do, they don't know what they're expected to do on that settings page. They are trained _not_ to bypass security.
