When you use a lot of FOSS, I think (at least for myself) I tend to pick my spots to get closed-source, for-cost software. If it'll run well on my platform, and I like the organization developing it, it feels more like... monetary collaboration, say, than simply buying another thing. If you're used to buying all your software, paying the minimum might look like just the (perfectly reasonable) cheapest way to get it?

Not to suggest that there actually IS any difference between the two groups when they pay - I just feel like more a good-deed-doing partner when I voluntarily pay more for something like humble bundle, that's all. It's silly and false, but it's definitely a feeling.

Or maybe it's because Linux users tend to be software developers, who are mostly quite well off due to the industry right now.

That and the fact that a lot of people want Linux to succeed as a desktop platform.

I pay for bundles I don't even play, just to get the word out that there is a market, and I want more games in it.