[jjgreen]

That's a misunderstanding I think, dictionaries say (of opt-in)

- choose to participate in something.

- to choose to be part of an activity, arrangement, etc

- The property of having to choose explicitly to join or permit something; a decision having the default option being exclusion or avoidance; used particularly with regard to mailing lists and advertising.

[the first few hits on google]

[mikeshi42]

Ah you're right - I never noticed the nuance of choice being a key part of opt in, rather than the participation itself. TIL!