Probably because civil discourse is important in a functional society and at worst this situation was a ignorant micro-aggression that could have been resolved in a manor fitting of the infraction. In this case, a simple reprimand and education would have been enough and more beneficial to the situation.
No one is really saying that the Clerk did nothing wrong. She did. The question then stands what was the severity of what she did and what should be the proper social discourse should be applied. If the Clerk had called the child a slur or some blatant shit I REALLY don't think anyone would be saying ANYTHING in this thread defending her. Rather, some believe that the punishment did not justify the crime.
I also don't understand how everyone in this thread somehow expects people to know traditional garb when they see it. I know I personally have never seen that article of clothing, nor the design of it pretty much ever, much less in a western country. Being confused is kinda normal, and the expectation that people HAVE TO KNOW just because we have all the info in our pockets is being a bit on a high horse. If nothing in her life has ever asked of her to know of such information, then why should there be an expectation of her to know it? If you are pissed at that prospect, then go your congressman or protest about the current education system and its poor representation of the clothing and of the people. (Again, just so that people don't put words in my mouth, the Clerk did wrong. I'm just saying that a lapse in knowledge in and of itself shouldn't be demonized)
For some perspective, I'm Canadian and I sure as hell don't know every kind of traditional clothing Aboriginal People had, nor any French or Metis clothing. Am I expected to?