Because they say so in the manifesto. They feel pity and respect for people that build an open source project. Without Open Source: No MacOS in it's current form, no php (facebook), no ruby (twitter), no linux (google), no apache (large parts of the web). Granted, there are paid OS-developers nowadays, but most is still for fun and passion and not paid for.
Which is precisely why open-source programmers have to be 501'ers. If we stayed late every night at our paid jobs, how would we ever do our unpaid jobs?
