I think it is important to consider several issues of a larger order here.
First, why is this news? How did it become news? What are the cultural and intercultural issues at play here which make it newsworthy?
My impression was that it was because it involved a young child.
Why did the parent feel that her child's anonymity was less important than bringing this to a news outlet?
This is a very good point! Usually parents are screaming that their child's privacy was violated.
Would a Black family have had more awareness of the taboo of wearing Greek symbols and signs?
I don't know.
Why is the taboo around this seemingly de-emphasized?
I think because it's just a shirt (yes, it's just a shirt) and it involved a child.
What role do the norms around predominantly white GLOs play in our own opinions about this? Example: Your "new members" can usually wear letters, but ours cannot.
I don't know.
I am not saying that the teacher was correct in her execution of this, but
she's entitled to be offended by something that is important to her. A note home without the wardrobe change might have sufficed. Very good points. I think the note home would have more than sufficed.
My opinion is that a lower class white woman was offended by an upper-class Black woman exerting power over her in a fairly embarrassing way*, and decided to go full throttle on the retaliation by involving the media.
I don't know.
This should not have been news.
I used to work in print and broadcast media. Quite honestly it was probably a slow news day. 
*and this has happened so often over the years on this very website, I know exactly how this conversation will turn out lol