Jeff - your suggestion says that all paint supplied at the CC should be sold through one combined paint booth run by the Millennium Series/organisers, at a price set by the Millennium, yes? That's how it reads anyway.
Lets look at the scenario of a typical decent level domestic team. Say that team A has a paint deal with Company X, where they usually shoot 10 boxes every month & therefore get £2 off the set price for a box plus 2 free. They may have saved up their free paint, and they may even have bought their paint in advance to bring along themselves (though with only limited parking per team this would be a pain) but they run out. They would then have to pay the full set event price for their paint, missing out on their loyalty discount. And if they then qualify for the Sunday Club their paint supplier may well give them extra free paint (I know this is quite common) for the semis & finals - would they then have to pay for this up front then get reimbursed by the paint company after they have got the money from the booth in turn?
The only way round this would be for the paint companies to give the organisers a list of which teams get what discounts or free allowances. This is hassle, and I don't see the MS manning a booth with complex pricing structures for different teams without having to charge for it, as the staff would cost them money. The paint companies could use their permanent staff, who may well be salaried, and it therefore wouldn't cost them anything in staffing.