The terms "opt-in" and "opt-out" indicate what the default is, so "... by default" is redundant. "Opt-in" means that you can opt (choose) to be in while the def

The terms "opt-in" and "opt-out" indicate what the default is, so "... by default" is redundant. "Opt-in" means that you can opt (choose) to be in while the default is out. In this case, since the default is in unless you opt out, it's opt-out.