Why are some Mexicans offended by gringos wearing sombreros?

DesiBeautyLover
I saw a sign at my school that said, "My culture is not a costume. This is not okay."
If it's offensive why do they sell them at every souvenir shop in Mexico, and why do they have those cutouts with sombreros where you can stick your face in? And why do Mexicans themselves wear them for festivities?
Why are some Mexicans offended by gringos wearing sombreros?
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