I don't think any restaurant agrees with "kwa" instead of "na"
It is one way of using the word "kwa". Here's an example from Dictionary of Standard Swahili: "Alikula wali kwa mchuzi".
It is. But tell that to the restaurant owners in Arusha or Dar.
Sorry, wali na kuku au wali na mchuzi - not kwa
I agree with the usage notes, below, even though "kwa" may also be correct.
I also heard "na" always. Locals never used "kwa".
Why "kwa" instead "na"? And, if it is not the way you speak any place, where should I use it?