I did some work for Halliburton in a past life.

Most of the people selling LNG for instance, do not have any control over the definition of a "cubic meter". Even so, none of them cheat, because the US for example, very much does have its own definition of a cubic meter and it isn't going to pay you a penny more, nor a penny less, than what that cubic meter is worth.

All that to say, you could probably try to cheat the system, but I'd imagine the people in Sumer and Akkad had what they considered to be a precise unit of weight with which to measure your delivery. It doesn't matter what someone in Mohenjo-daro said, you were only going to get a certain amount in trade for your freight in Sumer. So I could see a centralized authority for weights, (the customer), at the same time as having no one in charge of that unit of weight in Mohenjo-daro.

I could see people agreeing to it essentially because that's all you're getting paid for. Because I saw the same behavior long ago at work with Halliburton.