It's along the same lines as the word "faggot" was about a generation ago. To me, that word is as jarring as I am sure "spaz" or "spastic" is to people with a British upbringing.
That said, I personally don't use the term "spastic" any more (not that I really used it much before immigrating) even to Americans. It's not that big of a deal for me to drop it, but I also understand that there is a cultural difference in the meaning of the term. Hasboro obviously didn't meant to use it in the British sense, just like I don't think Mr Brains meant the derogatory meaning behind their "faggots".
That said, it is one of those things like knowing the difference between fanny and that "bloody" isn't a cute, mild swear word. It's required knowledge for people wanting to get along here without offending people, just as British people probably shouldn't refer to meatballs or cigarettes as "fags" or "faggots" in the States. However, we've as much right to have understanding when it's used in our own country as Brits use terms in their country that might be offensive to our ear, but had a different intention due to the difference in dialect.