The Express version restricts distribution of the binaries created with it IIRC. Also it's quite a heavy download compared to MinGW.
Please keep in mind that I'm not comparing writing C++ programs with MinGW vs. VC++. Rust just needs a linker for some tasks, which should be lightweight.
No one is suggesting that it should be the exclusive supported toolchain.
It may strictly be a larger download than MinGW, but it's certainly less painful and confusing. Sure, Rust may just need a linker, but MinGW is anything but lightweight.