There is a difference between the NSA being able to trace your identity vs. a profit-driven corporation being able to connect your identity to your online speech, location, and contacts. The first is governed by due process and need-to-know, the second is essentially public knowledge if the ad-tech revenue is sufficiently tempting.

Even in a pseudonymous environment where the venue operator presumably knows who you are (like HN), the assumption that other users cannot easily trace your words to your IRL identity reduces friction in communication. Environments where everyone publicly posts under their real identity, with those posts retained forever, tend towards low-information social signaling.