I ask because while I share your concerns (perhaps not to the extent you do) sitting on HN agreeing with each other does little, and neither does cancelling our accounts. What you'd need to do is explain to a vast number of users why this is problematic in very clear terms, and I'm yet to see anyone actually do that.
I'm willing to bet that many "anonymized" datasets are of the kind which could relatively easily be deanonymized, especially datasets which are frivolously used to make ads "funny".
> what is harmful about using anonymised data this way?
Whether it's harmful to use such data when it's not anonymous is a different discussion.
Dear person, who watched $show $number of times in one $time, $snarky_comment?
Netflix themselves might know who this person is, but there's no way anyone reading the tweet is going to be able to link it back to a specific person. Are you concerned that Netflix can query their own data?
And to that that Netflix was condescending and quite frankly rude.