They're actually two different things. St Stephen's Day is the 26th, but Boxing Day is the first non-Sunday after Christmas (although that wiki article seems to contradict itself on that topic).
Also, it's officially St Stephen's Day here, not Boxing Day, so if we take it that we can call it what we want, we may as well start using American spellings or other stuff just because we can. Boxing Day seems to have traditions in the landed gentry, who'd be rather more English than Irish.
None of which really means you can't call it Boxing Day if you want. But it does seem that, in this country at least, St Stephen's Day is clearly the correct term. For whatever that does.