By costume i meant official dress, at least that's what it means in French.

You might not be aware of it, but some companies provide allocations to locals and/or provide the products for them to wear. They are encouraged to show who the distinguished natives of this land are, which is quite normal. FYI, same companies also provide a subsidy for housing to locals. There are many things you will learn about as you move up in the hierarchy.

If westerners wore like locals or vice versa, then the true image of native representation would have been tainted.

Moudir