Software development may be open, and the result shared with an Open Source Software licence, but the actual hours of programming are not gratis.
Just like everybody else, I need money for mortgage, kids and food, money I make by doing things with computers, for people who are willing to pay for that.
One of the things I do for money, is develop Varnish, and the Varnish Moral License is the vehicle I invented for the paperwork.
The way it works is very simple: