There's a difference between the unversioned MSVCRT.DLL and the versioned MSVCR100.DLL, MSVCR120.DLL, etc. libraries. MSVCRT.DLL is technically private API, for the Windows OS's own use, and its API changes on OS releases. The versioned MSVCRnnn.DLL are intended to be shipped with applications, and that's what's in the download links you provide.

https://blogs.msdn.microsoft.com/oldnewthing/20140411-00/?p=...

The trouble is (AIUI) that the GPL has an exception for system libraries, and MSVCRT.DLL is shipped with the system and MSVCRnnn.DLL are not. Microsoft legally allows you to redistribute MSVCRnnn.DLL, but does not provide source. Therefore, distributors of GPL apps linked against MSVCRnnn.DLL have no way to satisfy their legal obligations. So MinGW makes life easier for GPL apps, which are a significant part of the target audience, by linking against MSVCRT.DLL instead. This is worse from a software engineering point of view because you're using private / unstable API, but it is not illegal, and it lets you avoid the GPL issue.

The program that put MSVCRT.DLL in a system folder wasn't being rude; it was the Windows setup program itself, which was perfectly allowed to put it there. MinGW is being rude by looking for it. (However, if an app linked against one of the versioned libraries did not ship with the versioned library, then yes, it would be expecting some rude app to have copied the versioned library into the system folder.)