I'm not convinced it's opt-in. I just visited that page, and everything was switched on. I can't profess to having a perfect memory, but I can say that opting in would have been pretty out of character thing for me to do.
I in fact explicitly remember clicking no to a prompt similar to this one, so I'm very puzzled how I got auto-opted in anyways..
> Also use Google Account activity and information to personalize ads on these websites and apps and store that data in your Google Account
Should be off if you didn't opt-in though. I just tested it with a new Google account and it was off after I opted out.
"Not opting in" implies you are are opted out by default. Thus everyone's negative reaction.