Probably because Microsoft stopped the insanity at examining the whole file for character set instead of just examining it for the content type. Not to mention the difficult in trying to figure out the encoding automatically. There is a big difference between "this is invalid UTF-8 so it must Latin1" and "this weird stuff must be EUC-KR".

Not to mention, saying a file is Unicode does not specify the encoding. There are multiple encodings for Unicode, and most non-Unicode encodings can be mapped to Unicode, as long as they are declared.