Although it's a glaring error to the few of us who can still spell, I have been unable to find any official mention or even gossip that explains it. Therefore, I can only speculate.
1) The characters are referred to as "The Basterds" within the movie. Perhaps the producers felt that the misspelling was comedic enough to justify being used in the title.
2) There are many places where a movie with a impolite word in the title cannot be advertised, so the producers thought that misspelling of "bastards" would get past the censors. (It didn't work; in England, the movie is being promoted as "Inglourious")
3) Since the title and parts of the premise have been lifted from Enzo Castellari's 1978 film, they altered the spelling to distinguish the new film from the old one.
