I have to disagree, Tom
It's not about "a plausible mechanism for transmitting a deliberate pattern and policy of racism from the central offices".
It's about management being aware of instances of racially intolerant behaviour occurring in restaurants, and not doing a damn thing about it.
Even if it's only 2-3% of the employees who're occassionally engaging in such unacceptable behaviour, it's the responsibility of management to observe, take note, hold employees accountable, punish when appropriate, and apoligize to those who are offended.