I have to agree as Gates wasn't arrested for being there, but for disorderly conduct, which may have some legs, but it is a catch all charge. If he was disruptive maybe, but he was on his own property, but for calling the police racist is a stretch and probably inappropriate on it's own. The police though certainly could have apologized and left him though after they discovered he was at home, regardless of Gates' response. But it looks like both parties decided to prove a point and here we are...
As to "us" well, that's suitable vague, but I can see Obama meaning it in the global sense rather than the racial sense seeing as he was addressing a large crowd or reporters.