I know a guy who once worked as a butcher and he says deboning chicken legs is a giant bitch. This guy worked in the meat section of a grocery store, which is nowhere near as chained as a place like Burger King. Nobody wants bones in their sandwich so deboning the legs is probably too much work. They take what is easier to debone.
The guy who worked there also said deboned chicken is much more expensive then boned chicken because of the work it takes to take the bones away.
(sorry, I have no idea if “debone” is actually a word)