Its pretty much impossible to know whether or not such a contract existed because a lot of this stuff is kept behind closed doors. That being said, I'm gonna play devil's advocate here.
If that exclusivity contract did not exist, why was Kings Dominion shunned completely for a brand new B&M for nearly 20 years?
There's plenty of reasons why.
1. Kings Dominion wanted a coaster that B&M just didn't provide. Note that every single coaster built under the Paramount era was a coaster that isn't in the B&M catalog. You have multiple launch coasters, a wooden coaster, and a wild mouse.
2. They wanted something unique. With the exception of Stunt Coaster and Flight of Fear, most of the coasters at KD (built within that time period) aren't found anywhere else.
3. They wanted a boundary pushing ride. Even though B&M did try to push themselves in the 90s, they sorta scaled back in the 2000s. Going back to the first point, KD wanted something that went beyond what B&M was comfortable with.
Why was Kings Dominion shunned when they have a larger attendance share than Carowinds?
KD, KI, and CW were all "shunned" when it came to getting B&Ms. It wasn't until the late 2000s that they all got B&M coasters. CW has the most market share of the former Paramount parks and all it got under the Paramount era were common clones and a few amazing looking flats. It wasn't until Behemoth that they started getting blockbuster custom rides. KI was the last Paramount park to get any B&M coaster.
So that leaves KD getting the used coaster. Well... It's cheaper than building new. Why spend the extra cash building a brand new roller coaster when you can move an already existing one and build it for a fraction of the cost? You save quite a bit of money for future projects that way.
If there was no exclusivity contract of any sort, why did Busch almost solely focus on B&M for over a decade?
Same reason they stuck with Arrow for so long. The rides are reliable, have high capacity, and are very safe. If Drachen Fire wasn't such a bust, chances are you'd see a few more Arrow coasters pop up at the Busch parks as well. I'm sure that's one of the main reasons Six Flags Great Adventure and Great America are always clamoring for their prototypes.
- Having such a contract also gives the competition a chance to 1up your projects. Not sure if that's a good idea.
- Not saying such a contract didn't exist, but there's plenty of arrows pointing to KD not getting a B&M out of coincidence.
- I do recall KD releasing an online survey for future projects mentioning an Addams Family-themed dive machine. Not sure if that has any bearing on the contract. Needless to say we all know how that turned out.