Today I started working on altering a kilt made by a well known tartan mill.
Out of curiosity, why was it sent to you for alteration? Was it needing to be taken in to a smaller waist size? I'm wondering if the stretching action from having no stabilizer is part of the reason it needed altering.

Anyway, I would guess that there are a LOT of kilts out there lacking proper internal construction. With the proliferation of budget kilts and the steep competition to sell them to tourists and entry-level kilt-wearers, suppliers are taking shortcuts in the hopes that those who buy them aren't ever going to know the difference. Some may be doing it in a real attempt to defraud, while others probably don't even attempt to claim their kilts are too well built. But the problem is, the general public doesn't really pay attention to this sort of thing.