It's named like this because it is a subsytem of the NT kernel (hence the Windows subsystem part) that implements Linux system calls to run Linux applications (hence the for Linux part).
Naming it the other way around ie Linux for Windows would imply that the Linux kernel is involved which is actually not the case at all.
Also, they previously had a Subsystem for Unix (SFU/SUA), so they're being consistent with their naming. That was the old POSIX layer that included, IIRC, a shell script to wrap cl.exe to take gcc-like arguments, and you can get third-party compiled binaries of things like bash.
Naming it the other way around ie Linux for Windows would imply that the Linux kernel is involved which is actually not the case at all.