Requiring it to be open source is not just about trusting the publisher. There are a bunch of other possible reasons, including wanting to support open source as a counterbalance to proprietary software.

For me, it's a big deal (although not a dealbreaker) for that reason. If I have the option of two different pieces of software, one being open source and the other proprietary, I'll choose the open source one every time unless there's something really exceptional about the proprietary one. But that's very rare.

I was just trying to think of any proprietary software I use outside of work (where I don't have a choice) or games. There must be at least one, but I can't think of what it is.