If Kuat Systems Engineering built the A-Wings how the heck did the Alliance get their hands on them ? Did they steal them all ? Kuat built ships for the Imperial Navy, ships that were used to fight and kill rebels. I find it hard to believe that Kuat would sell ships to the Alliance and that the Alliance would be willing to buy ships from them.
Technically at the time they start showing up now (Rebels TV series) there is no formal "Rebel Alliance" it's just a bunch of small random covert group fighting for individual reason.
In the case of the A-wings we see in the series specifically, they were likely purchased legally by the Alderaan government, then just dropped from the books, or somehow written off.