Which is English.
American-English is a dialect which is rife with mispronunciations and mis-spellings. "honor", for instance, should be "Honour". The dropped 'u' is just plain lazy.
"Lazy"? Try "efficient." You don't pronounce the damn letter anyway, so why throw it in there? We're trying to make your language a bit more sensible. 
For the oblivious, that was sarcasm.
Point conceded

Well, actually, the U is somewhat pronounced. It implies a different nuance of pronunciation than simply 'or'. /EnglishTeacher
Well, that's what any British English teacher will tell ya, and it's true. You just have to hear a brit and an american say the word.
But, I'll agree that DefenSe and DefenCe is pretty much interchangeable, but deFENCE makes more sense, as that's talking about a barrier.