Each culture puts its unique stamp on the fashion world. Many people tend to regard fashion as "just a bunch of clothes," but if you really take some time to research the history of fashion around the world, each culture throughout their history, has put itself into every fiber of the fabric that represents them. The history of kings, queens, wars, poverty, power, love, religion, historical movements, holidays, everything is right there in every button, rivet, thread, zipper, and ribbon. So what happens when someone outside of a culture decides to take many of these intensely personal, religious, or culturally significant pieces of their history, and parade them around in a what some feel is a racist, cultural inappropriate, or insensitive way for so called fashions sake, or as Halloween fare?
What is cultural appropriation? It is the adoption or use of the elements of one culture by members of another culture. Cultural appropriation is sometimes portrayed as harmful, framed as cultural misappropriation, and claimed to be a violation of the collective intellectual property rights of the originating culture. Often unavoidable when multiple cultures come together, cultural appropriation can include using other cultures' traditions, fashion, symbols, language, and cultural songs without permission. According to critics of the practice, cultural (mis)appropriation differs from acculturation, assimilation, or cultural exchange in that the "appropriation" or "misappropriation" refers to the adoption of these cultural elements in a colonial manner: elements are copied from a minority culture by members of a dominant culture, and these elements are used outside of their original cultural context—sometimes even against the expressly stated wishes of representatives of the originating culture.
To the people affected by such misuse of their culture, it is someone making a complete mockery of things that many consider sacred or personal to their culture usually by someone seeking to make a quick buck off of their culture, without understanding any significance of what they are wearing. Typically, from the outside looking in, it's hard for people in disagreement to understand how this makes those affected feel. It would be like a stripper in a nun's habit stripping on cross with Jesus nailed to it. These symbols and the meanings behind them are sacred and to have someone, especially in a movie or music which is widely spread throughout the world and internet distorting its significance, and having people use what's important to them in a sacrilegious way, is hurtful to that culture because people tend to then treat it as a joke.
This becomes more significant because you have someone in power, saying I have the right to steal from your culture and use it for my own personal gain. These people can then mistreat and abuse it however they want without a thought or care to how it is affecting others. This is not the same as cultural exchange or assimilation by one culture of another, because the person doing it has no desire to know or learn why what they are doing is inappropriate. They simply do not care. It's yet another way in which certain cultures and people are being "put in their place," by the larger or more powerful persons in society, because it says to them, they can come in to your world, at any time, and anywhere and take what they want without your permission and what are you going to do about it.
To be clear, this is not to say an individual should NEVER where anything from someone elses culture. That would be ridiculous. It is to say, that asking for permission before you take as a designer, learning about why the clothing has it's significance for that culture, not making a mockery or racist event out of wearing them, or flat out not wearing certain things because of the meaning behind them and the inappropriate nature of wearing them all go into it. To think of it another way, you wouldn't want some random person snatching your wedding dress and go off parading in the mud with it, and then hand it back, like, oh well.