They clearly categorise their jobs as "for native and non-native speakers."
Native-language fluency isn't the same thing as race.
There's also a very understandable reason that a company would choose to reserve orders for native writers. You don't have to look any further than this forum for examples of ESL writers who think their horrible English-writing abilities are good enough to write professionally in English despite being unable to put together even a few simple English sentences in their forum posts without mangling the language. Even most of the very few who might write fairly well in English are still recognizably ESL. No native-speaking customer wants to pay hard-earned money for that, especially, without agreeing to accept work produced by ESL writers. In the last 10 years, I've encountered exactly THREE whose writing in English is passable as native-English writing.
What's so hard to understand about that?