Because I would expect that you generally sacrifice part of your salary for the ability to work on free and open source software full-time, much like you would generally take a pay cut working at a non-profit.
That's senseless. Non profit doesn't mean "worthy cause I should want to die on my sword for" it simply means the corporate financial structure doesn't accommodate profit-sucking shareholders, nor building the corporate coffers from year to year.
I don't know why you're being downvoted. I'm a contractor and my hourly rate absolutely depends on how good I feel about a project. Working on (useful) open source is a plus. I wouldn't go down to $20 though.
