Not to argue, but -
Taking from Nintendo technically is stealing from another spriter as well. After all, some sort of spriter or artist had to make the sprites for Nintendo, right? :P
It's someone's hard work, anyways. Those sprites just don't appear out of thin air. Though, yeah, maybe they don't care too much about usage, or, like Sapphire_Dragon said, they would've put up some policy regarding them. Sorta like with fan art and whatever, I guess - they know it's there, they know what's happening, but don't do anything about it (unless monetary gain or potential reputation damage is involved).