Your statements demonstrate a misunderstanding of WINE and APIs. What you're getting with WINE is a community implementation of the programming interface that is accessed by the instructions in the Windows executable files. It's using the native operating system, and not involving a contract with Microsoft. Similar reimplementations of programming interfaces happen all the time in Linux with different binary formats (with ELF being the most common).
The $25 pays for the game, not the platform. If other platforms become available in the future, I expect - as has been the case for all other games I've looked at here - that additional downloads for those platforms will be available at no extra charge.
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. If this game gets popular enough, the developer may decide to add additional platforms, or hire someone else to do that porting.
In the meantime, we have WINE, which gives us a way to play this game natively on a Mac or Linux system without needing any additional effort on the part of the developer.