Not sure what it is about the construction industry and getting paid for the services that are being supplied. If you take your car into the shop to get it fixed or if you see a doctor and they hand you the bill, there is no argueing about it. You pay or else you will not get your car or they hire a collector.
But with building the customer, even when they have agreed to the terms in the contract they still decide that they need to knock some money off. And the worst we go along with it. What a sad world.