There are other types of mustard that are yellow, so where these are available having one type of mustard called 'Yellow mustard' doesn't really make sense, hence 'American mustard'.

Not sure of the other mustards they have in NZ, but where I live (UK) the main mustard, aka English mustard, is also yellow (and hotter and better) so we'd typically also call the other one 'American mustard' (even if the bottle itself says 'Yellow mustard').