That doesn't really follow. I can easily answer:
Why would someone agree to [program] without any compensation?
It's fun, and adding/fixing a feature for everyone means they might add or fix a feature for me.
What I can't answer is: Why would someone agree to [do data entry/handle customer support/agree to be legally liable] without any compensation?
Those activities aren't fun, and if I handle customer support for you there's nothing in it for me.
Further, a lot of open-source programmers are getting paid. See [1] for Linux, the Mozilla Foundation for their applications, Joyent for Node.js, &c.