Interesting point. I know of a story where a couple were sentenced to several years in jail for kissing each other in a sacred square in the Middle East. Is it normal to kiss somebody in public, or is it normal to sentence somebody to several years imprisonment for it? Is normal different depending on location? I agree with you that he should have known better, but you can't expect a visitor to the UK to know of all the cultural delicacies straight away. If you think otherwise, it would be interesting to see you abroad, interfacing with the locals when you are on a holiday. How often will you unknowingly have "insulted" somebody because of a lack of knowledge of their culture?
He's been here for 18 months though.
Personally, I don't think that calling someone a nigger is that bad, especially when it's done by someone who isn't a racist. However, there's been enough fuss about it, that if he's found guilty, he can't stay, which is a real shame.