In the cabinet-making business (where I'm currently employed painting kitchen/bathroom/corporate cabinetry at a furniture polishers) wastage does apply to labour. The amount of time to re-cut a piece, re-sand a piece, re-paint a piece, is 90% labour/time and 10% materials. There's various reasons for it, but as an example, 3 pieces of dust in a satin finished drawer front means a rework. 3 pieces of dust in a gloss finish means the extra labour for a polisher. 3 pieces of dust on a high end clear-coat, rework.
Lump in the material (common in MDF, melamine products), re-sand, re-prime, re-paint. Or rebuild.
Dust, bad materials, cutting faults happens, who pays? Because if it's not charged to the end user, everyone just chases it down the chain until prices get higher at the supplier end and, as it's impossible to keep things 100% perfect at ANY stage, customers will pay more than 10%in the long run.
I know of a company with a $100,000 paint matching machine, and they WILL NOT guarantee a perfect match, due to variance in base products.
Lucky you're not in the glass industry, they account for 20+% in breakage, and charge accordingly!