First and foremost, they've said on more than one occasion that the movie isn't based as much on history as it is on the graphic novel by Frank Miller.
Second, who has standards of what Persians from 400 BC looked like? The lines that are so clear and visible now thanks to the media weren't even in existence back then - how can one throw a fit about the accuracy of how people from well before our time even looked?
When Jurassic Park came out, were archeaologists throwing tantrums about what T-Rex should look like? No, because it's ALL speculation.
The Persians back then vs. the Persians now are two very different things.
