I'm reasonably sure this isn't the case. Even in pure math, postdoc salaries tend to start at $50k ($45k in low cost of living areas like Urbana Champaign). Associate (tenure track, but not yet tenured) profs tend to start at $70k. I can guarantee you engineering/CS makes significantly more.
However, the average professor makes very little, because the average professor (of, e.g., Medieval English Literature) has few marketable skills.