The Concise Oxford Dictionary effectively answers your question when it classifies 'mathematics' as 'noun plural (also treated as singular)'.
Brits regard it as a plural, and hence retain the 's' on the end of the abbreviation. Our American cousins treat it as if it was singular, thus disregarding the fact that 'mathematics' ends with an 's'.
Sherrardk:
When I was teaching I annoyed my head of department by removing the sign which said "Mathematics Department" and replacing it with on which read "Department of Hard Sums" ;-)
I just think it's a little self centred for us to think that one is correct and one is incorrect, traditionally we would use maths and the Americans would use math, that doesn't make either way wrong.