This really is the key question. When installing an app, a user should just be able to breeze through the install wizard and not waste time on every screen reading to see if it may slip an unwanted install in.
About 95% of installs are from users who don't understand what they are doing. Of course they are going to make it opt-out by default. They will probably provide an option for advertises to choose opt-in, but that will be used in only about 1% of the cases since it cuts their profits by 20x