It's usually not, but there's a certain logic to it: they valued your work high enough to pay you in SV, why would the same work be less valuable just because you moved offices?
Totally agreed. I keep trying to explain this to companies as an interviewee, but it does not go well. Even in my current job, doing the same thing/title/exp/degree as others on the team, I am paid less; because I work remotely. Yet they do not expect less of me than my coworkers, but they pay me less due to geographical region.