When the askers (mostly non-native) prefer answers from native English speakers, there could be a reason. For instance, if I'm working with a company that deals with clients in the US or UK, it's important for me how native speakers would address the concern. I then learn and apply it in my practice.
Another reason is the cultural difference which is, by no means, difficult to learn through online sources including mainstream media. My many answers/questions have been left with comments or remarks that 'As a native, I won't prefer this' or 'That's what looks natural to my (native) ears'.
When I was pretty new to this, I reacted to such comments too quickly. But then I understood that it's important for a non-native person like me to understand and learn what's natural. I accept that I still struggle with what's 'Indian/Asian' English and whether it's valid/accepted. For instance "I have done/did my education/degree from XYZ university" is absolutely fine with many Indians, but it's an improper way!
So, if someone has asked something to native speakers, and if I write in an Indian way, I'm afraid that won't serve the purpose.
Moreover, we have a wonderful system of
tags. And, I remember that someone created
Indian English as a tag after the question was raised by me on this board.
So, to conclude, it's justified that if someone is asking to native speakers, natives take on it.
An extra but interesting note: when non-native write improper answers, the native speakers here correct it. And, when native speakers write it incorrectly, again the native speakers only correct it. So, ultimately, with the native speakers here at the helm, I think we are in good hands.
I don't say answers should be restricted to a specific group of people. No. Certainly not! Because an answerer like me always respond to such questions like -
Though I'm not a native speaker, IMO...
You, being a native speaker, may not have an idea, but it's quite natural for us to seek natives' help. No harm in that!