MinGW chose MSVCRT.DLL because it meets the definition of "System Library" referred to in the GNU Public License. There is a special exception that GPLed programs can be linked to a proprietary, closed-source component if it is a system library; i.e. something always installed and always present on a certain type of system to which that program is ported. Without that, you couldn't have GNU utilities as replacements for Unix utilities on a proprietary Unix, linking with its libc.