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?

You can have balance in your life and be a good coder. I think balance and passion are mutually exclusive, though,

I don't think balance and passion are mutually exclusive. That would be conflating passion and obsession.