Since the private sector/industry work is competitive, it is considered market rate. The reason that post-docs are not considered market rate is that the institutions do not compete with each other, and the rate is determined without concern of the applicants.

Both. You have a choice between:

A. $100k in cash compensation, or

B. $40k cash + a shot at becoming a tenure-track professor.

The market is saying that opportunity to possibly become a professor is worth $60k.