[dragonwriter]

> Because they were willing to work for $40k.

That doesn't mean that they weren't providing $70k of market value, unless you assume that (at a minimum):

(1) they were maximimizing their pay in choice of job, and (2) they had perfect knowledge of all the alternatives jobs that they could otherwise have obtained.

These are the kind of assumptions that are typical in Econ 101, but to which real humans do not actually conform, particularly the second.