It's more likely that they accidentally opted in, various stories about this sort of thing have been debunked to accidental opt-ins.
I used to doubt too, whether it happened on its own or whether people made mistakes. Even back then, I believed if so many people can be made to make the same mistake, then it's more like they've been duped than that they all just happened to make the same mistake. Now, when I saw it happenning right in front of me, I was furious.