Not every non-US developer have a credit card. Particularly if you are talking about young students.
Windows SDK (That means: Compiler, Linker, Libs, Docs and Headers) is a free download from MSFT site. You only pay if you are a masochist and want to use Visual Studio.
Or ok. And what about your dependencies? Let's say you have a dependency on a library that is not shipped with OS X, what do you do? Link it statically and see your tiny gem becoming a monster of a few megabytes? Or do you simply link it dynamically hoping that somehow there will be a binary with the right version lurking somewhere for the user to download.
Yeah, that's what happens when you develop OSS for windows. Or do you rely only on native windows libraries that you know will be there for sure, or do you have to compile against the latest version of the library you want to use that happens to have a binary available for windows users to download. Or then, you resort to some strange things like cygwin, MinGw.
But didn't we all complained about that when windows was the only somewhat userfriendly OS out there? Didn't we all complained about how much it sucked that you couldn't have a free C compiler chain on windows. Well, now at least on windows you can download the Windows SDK and build code without Visual Studio. If and when Microsoft Windows C/C++ compiler, linker, docs, libs and headers are updated you can have a fresh and free Windows SDK download without having to shell out money to buy Visual Studio, would it be so difficult to apple to unbundle GCC toolchain from XCode and make it freely available?