I don't think this is a decision that the Scottish people sat down and contemplated based on business models. It is more of a cultural mindset, and is not controlled by any one person. It is a cultural backlash against the revolting displays of the tartan tat merchants who are trying to make tourists shell out every last dime for an icon of Scotland, and making everything kilt- or tartan-related seem foolish.

For a reputable kiltmaker to engage in behaviour (and dress) that would associate him with that sort of business model would be disastrous to his reputation. This is not to say that having shop employees wearing a kilt would necessarily destroy the reputation in and of itself, but again, it's more of an overall mindset. Were it not for the tartan tat clowns, I would bet that local kiltmakers would have no problem with their employees wearing kilts to work.

That's just my take on it, anyway. I could be wrong. But even if I'm right, it doesn't necessarily have to be logical. Cultural backlashes seldom are.