Weight is a lot easier way to scale when you have tractor trailers coming in every day but with all due respect I have never heard that selling by the cord is illegal, and most firewood is sold that way.In the State of Maine, it is against the law to sell wood by the cord. It either has to be by the Load, or by Weight.
This presents a problem because most people want to buy x-amount of cords of wood. We cannot sell wood like that, so we load on what the person wants for "cords" and then sell them the load of wood. We move so much wood though, that when we say it is...say...6 cords...it really is 6 cords. You just know how much wood is on the truck by how far up the stakes the wood sits.
Myself, I always thought buying or selling wood by weight was way off, so one time I cut and measured a load of 8 foot hemlock pulp. It measured up by the stick at 13.4 cords. After it rolled into the mill, the scale slip by weight said; 13.2 cords...2/10ths of a cord off what I measured...that was pretty darn accurate I thought, so I never complained after wood by weight after that.
On mixed loads, it gets a lot more variable, but it all equals out. If I cut a pure beech for a load, it will weigh up as if it is 11.2 cords, but if a load of pure popil goes in, it might only be 8.5 cords. Along the way, I have got paid an average of the two.
The only time it gets unfair, and that is; unfair to the paper mill, is in the Spring when loads of wood are teeming with mud. That really adds up, but then again, its nice to get a little more money for slogging through the mud. I really, really, really hate logging in the mud. But the paper mills know this, and could stop taking wood in the spring, but they need wood, so they buy it.
You are right about weight being variable... if it's a mechanical job in June and the limbing crew is 2 weeks behind the harvester, the trees will lose some weight because the leaves are still sucking the moisture out. Back when contractors were chipping in the woods and selling it as pulpwood the scaler would take a sample, then they would calculate the amount of usable chips and moisture content; then pay based on those numbers.
I cut a load of tree length fir pulp a few years ago, by the time that I got it trucked the stems were coated with ice. I got an extra 10% for that load just because of the ice... the trucker was lucky that he didn't meet the weigh wagon.