I suspect its because we americans tend to be incredibly egocentric about language. Everyone should speak english, especially in foreign countries where we don't speak the language and everyone should use our spelling because its the right way to do it. *g*

That being said, it took me awhile to get used to British spelling and my brain would at first just stop and say, whoops - typo before I got used to it. I have a tendency to american-ize spelling in something I've downloaded just to keep me in the flow but that's a personal preference and I certainly don't expect people to change "normal" spelling for my convenience. I do expect dialogue to be appropriate for the speaker but in BTVS that's often more "Californian" than "american" since I don't know anyone who talks like the Scooby gang.