So 'we have cultural differences' is racist and '[all] white people are ignorant of other cultures and even so ignorant they don't know how ignorant they are' is not.

If I must ask myself why such a blatant generalization and insult is offensive--does this work both ways? Shouldn't someone ask themselves why 'cultural differences...I can't do the job...warmest regards' is offensive--let alone racist?

Let me point out, "I can't do the job" and "cultural differences" are not insults. "Ignorant" is.