Comment by Paqi

People similarly dismissed concerns about information Facebook was sharing with third party apps that seemed to just provide quick entertainment like "the which celebrity are you most like quiz!" Years later, it turned out to be part of a massive data-mining operation meant to get information about voters.

You could point out reasons why this isn't the same thing, but you'd be missing the point. I, as an individual, can't fully assess the privacy impact of every single piece of technology I use, and what seems to be a benign feature backed by a corporation I trust could be anything but.

That being the case, we need to adopt certain heuristics to help defend our privacy. Granting as few permissions as possible for applications we use, and avoiding using a single identity across all of our devices is a good way to mitigate that risk.

Are you an individual that is willing to reduce your privacy a bit for added convenience? That's perfectly fine. But the ethical thing for Google to do would be to make this an opt-in feature, rather than an opt-out. The fact that they made this browser/Google Account sync the default was no accident. They want to gather more data on their users, and know if their users were stopped and prompted to think about whether or not they actually wanted this convenience, they would have less participating users. So they've made the unethical choice, and made this an opt-out feature.