It's a lazy use of language, but yes, "Why does <person-X> justify doing <thing-Y>" is the same as "good reason" in some colloquial uses of US English. Furthermore, in US English, just like in British English, there are plenty of class and education factors that influence how someone would phrase the statement in question.
Most educated coastal folks would not use "3 good reasons" in this case, but at least in my experience, in less educated communities in the South it is more likely to be phrased that way. I implicitly assumed that the teacher did not mean "good" pertaining to slavery, but that may be because I grew up in an environment with different colloquialisms than others. Frankly, if I were a sleep deprived teacher just trying to get something done, I could easily see myself making the same mistake and then seriously regretting that it warped my intention.
This is particularly interesting because I think that the use of language in this case actually reinforces the intent people read into the situation. With some people confused as to how this is "racist" because of a different reading of the language in addition to a different moral interpretation around the assignment.
