"Opt in" and "by default" are contradictory phrases. They mean literally the opposite of each other. The only reason it's coherent is that most native speakers will elect to believe "by default" was used correctly because it's the easier to use term.
> there is not a rule of the English language that would make their usage unacceptable
I think you're using emotionally inflammatory language, possibly unintentionally. no one is refusing to accept what they wrote.
That being said, the way they used "opt in" is categorically wrong. It's the opposite of the dictionary definition.
https://en.wiktionary.org/wiki/opt-in
> 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.