From my understanding it's more of a contract. The brands like having their equipment being present in any media as long as it is shown in a good light, it is advertising after all. But it being advertising, they want certain things and don't want others. Brands don't want their stuff in a game like GTA because it's controversial and has lots of crime and violence (not that I have anything against it, I love GTA), likewise they don't want their stuff to appear in a bad game (bad graphics, terrible bugs, etc..). They want their brand to look and work good within the media, although a vehicle in a game really does not represent it's RL counterpart, a bad representation could put off a potential buyer.
Sure none of this gives any reason as to why there is or isen't any certain piece equipment in the game, but there is a lot more to it than "they should be able to add it..." Giants could only put in so much stuff, as it was the game was pushed back and still released before it was truly ready, they deffinatly didn't have time to be adding in more equipment. So they themselves chose certain equipment to get in and some to leave out. Also the Brands have tons of say over what goes in, there could be any number of reasons a brand did or didn't want something in the game.
Neither Giants nor any of the Brands have released nor probably ever intend to release their license aggrements, so we have no clue what is or is not in them.