THIS MOVIE TITLE NEEDS A SPELLCHECK
Heh, this is like one of first things the actual movie touches on...
....Where Will Smith brings his son to the day care and complains that one of the words is a cuss word and that another on the wall, which is "happyness," is spelled with a "y" instead of an "i" to some Asian man -- who doesn't speak any English to him, either.
I think it was spelled that way in the flick, as a symbol to show that some people just don't care about anything. It's that whole "ignorance is bliss" thing this flick got going on, saying that people should care about things.