I started an entire thread about this in Suggestions and Ideas.
Those who think that because some people have the DLC and Some don't that if the Scenario then has DLC, they will be forced to buy the DLC or not play the scenario do not clearly see the point.
The idea is to design a scenario that is a level playing field for all. So when you see the results later from players Charlie and Tom, Tom who has the DLC, cannot put in a Ghost Ride in his version and show it off.
Or more to the point, when these Scenarios are done in competitions, then the players play against each other with the same building sets. No Munster Kart pieces showing up on the Main Street buildings. My Small Park Competition for RCT3 was famous and we used sets that all could have access to. The Scenario editor then allowing the creation of researchable or useable pieces only from sets that everyone had access to.