"The $25 pays for the game, not the platform."
Paying $25 or more to run a game that's not only a Digital Copy of the game but doesn't run natively on any platform is completely unacceptable.
"Developing for multiple platforms is expensive, difficult, and time consuming, especially when a game only has a single developer, and especially for Linux, with all it's different flavours."
∆This is a completely bullshit argument.
1) He used a Commercial Game Engine, using a Commercial Game Engine not only cuts down on the Time and Labour needed to develop the Game but also makes Compilation work relatively easy. Once you get a stable Windows port (in most cases) it's just a matter of recompiling the game again for Linux. It's not like he coded the game from scratch.
2) Expensive? No, that's why he used an Engine.
3) Yes, there are different flavors of Linux. However it definitely doesn't require recompiling the game over and over again for each Linux Distro. Maybe a Ubuntu and Arch Linux Compilation at most but that's it.