I'll give you another example.
A few years ago I was perusing the rules of a national Honda championship.
As usual with these trophies there were many sole suppliers. You got pretty much all the material (Power Commander, crash bungs, fiberglass fairings etc) straight from Honda upon paying the entry fee with the exception of tyres.
Tyres were restricted to a single Michelin model (fine with me) but you had to buy them from a single place nobody had heard from before. Needless to say this meant paying the tyres considerably (if not a lot) more than what you'd have paid them in any tyre shop, plus shipping fees. Of course it would have made much more sense to have the tyres available at each race at fixed prices but that way the outfit owner wouldn't have feathered his own nest.
That's how sole supplier rules work: under the thin patina of "fairness" somebody is getting fat at everybody's expenses. Don't you love when politics and racing mingle?