[kartan]

> documenting open-source code for free

Foundations that own the open-source projects are allowed to use volunteer work by law. Corporations with a for-profit interest doesn't.

> or answering a stack overflow question?

This one is more tricky. But it depends on who asks the question? It is an individual to get help on a problem? Or is it part of the ways of working of a company to avoid hiring experts? Evaluating intention it's usually hard.