This is usually not the case. I don't know of any "monopoly" law that allows competitors to hijack space on someone elses equipment just for the sake of variety. Coke or Pepsi puts up the capital expense in equipment to pump as many gallons of thier product as they can. This is clearly how they profit on fountain syrup. Any competing gallons kind of put that model in reverse.

That said, if the customer owns his or hers own equipment then it is fair game for whatever they want to pour. Also, some national contracts allow an allied brand such as dr. pepper or crush to have a valve on pepsi or coke equipment. That's how the taco bell one is, but it is also how the contract is negotiated.