It's a traditional source of friendly rivallry and amusement. Amercian English has quite a lot of different spellings to the English used in the rest of the world; this is mainly due to a linguistic "purge" a couple of centuries ago where all kinds of surplus letters were removed from American spelling. Hence, "honour" and "harbour" became "honor" and "harbor", "Pittsborough" and "Marlborough" became "Pittsburgh" and "Marlboro", and so forth. There are other differences, too - "amuminium" (pronounced "ell-yoo-minny-um") is known in the US as "aluminum" (ell-oo-min-um).

There are people on both sides who enjoy a bit of lighthearted mockery now and then. Well, I assume we're all being lighthearted, but you can never tell with some people...