If that's truly how people use it, it is very strange.
In actual roman numerals MM = 2000. Using 'M' as a roman numeral but then multiplying digits makes no sense at all (you'd need numerals for all the prime numbers to represent arbitrary numbers...).
And in SI, the prefix 'M' (mega) already means 1 million, so to me it seems MM is the notation that maximizes confusion.
I totally agree - there's not a roman numeral justifaction for it at all and it's very confusing in normal situations.
My understanding is that it comes from financial (specifically trader) jargon and I suspect it probably originated to differentiate it from some other use of "m", but don't know for sure... Maybe someone else knows why it arose?
I always thought it was short for "million monthly" so when you see something like "we have 10MM users" it would be 10 million monthly active users (i.e. 10 million users who have been active in the past month). I have no idea what it means here.
OK, so after a bit of searching, it seems that it comes from the Latin word for "thousand", millia[1]. And it's apparently common in financial contexts.
So 1M is 1,000. 1MM is 1,000,000. 1MMM is 1,000,000,000 (though the former and latter are not as common). Still seems like a confusing way to abbreviate to me.