Many sub contractors live on the mercy of the companies they supply. That forces them to show more goodwill than they want.
I remember a couple years back when Ericsson unilaterally decided that they would stop paying their bills after a month and instead changed it to three months. So, do you want to piss off the biggest company in the region or do you just say “Thank you, sir”?
As an aside, what kind of amoral sod goes around teaching companies what bills they can ignore and how morally bankrupt must you be to listen to them?