You know, I've always thought of "MM" as "millions," plural, the same way "p. 55" is "page 55" and "pp. 55-56" is "pages 55-56." No idea if that's common usage/interpretation though.
It's a common practise, but it's not a standard AFAIK. It's from latin mille which means 1000, so M=1000, and MM=1000000.
Unfortunately there is also a much more widely used international standard where k=1000 and M=1000000, so the M/MM forms are to say the least, confusing, and ought to be scrapped.
Is this just a standard/common practice in America? I have yet to see it used anywhere else but then I am primarily exposed to UK/European news sources.