So, in order for them to continuing giving your company free samples, you have to use the samples like they want or else they stop sending you free stuff?
So if Company X sends you a case of models, they want you to use them as per their wishes? Like for reviews and build ups, but not to give away as prizes or raffle or auction off? That is reasonable to me.
Building them yourselves is free advertisement for the product and harkens back to the days when five and dime stores had factory built up models as part of the store display. The build ups might spur a modeler to go out and buy the kit themselves.
Giving the kits away as prizes or selling them off via auctions or raffles takes away from any potential sale. Someone who might have walked into the hobby shop to buy the kit might not if they can take a chance on winning it or otherwise acquire it more cheaply.
Just seeing it from another angle.