I have had a very good look at various ref's and I can't find an answer. I can find plenty of US metar's that are using it. I used the NOAA translation function and it just ignores the MM. Must be a US thing although like you say NOAA and the FAA don't use it in their manuals.
Chance are that it is something very simple like how the TS were observed, an automated system perhaps?
Any US domestic dispatchers able to answer this one?
Someone ring the local met bloke.