In Detroit, there are several nail shops in our local malls operated by Asians, and their business seems to be steady and frequent. The manager uses the same tone of voice with all. Perhaps the manager used a patronizing or condescending tone of voice when speaking only to the African-American employees. The manager supervises many employees of varying ethnic groups, including several African Americans. I have worked with this person for years - the tone used is the same for everyone.
Expectations are fairly high, but the same is asked of everyone.
There is, however, one African-American employee who takes exception to the tone of voice. No others do, and this has not come up in the past. It appears that to everyone but this one person, all are being treated equally. I should add that this employee has had some extremely negative experiences in past employments - not at our company. I hope this helps.