Why is it? I don't know, you'll have to go ask 300,000,000 Americans why. You can do an ignorant black man's voice as a white guy all you want, it just makes you a racist or at very best a moron (in the mind of a huge majority of people). If a black guy does the voice of a square white guy - it's funny! I laugh and maybe it's just as racist but that's what seems to be tolerated in society.
Now i know FTL isn't a show to pander to the majority, but this isn't one of the things you can 'be an iconoclast' on. If i had a show to preserve i wouldn't dare hop on the air and make fun of black people with a thuggish voice.
In my personal life i do voices all the time and often they're spanish or black people, but on a one-on-one level i think it's fine when i know my audience. If a bunch of strangers were listening, it would alter my perception of what's ok to say or do. I think George just has too much free time on his hands and is looking for improprieties where their are none.