Excellent observation. If an instructor needs to have separate classes to simplify his job, then that should be clearly noted in the materials which should specify which type of class is being held on a particular date so students don't sign up for an autopistol class with a revolver, or vice versa.....there is a lot to watch and keep tabs on when on line with life fire. it may be just a simple case of needing to keep apples and oranges separated with separate classes or training.
In your example, for instance, the course materials should clearly state that the class applies only to vehicles with two axles and also that it doesn't apply to vehicles with less than 2 wheels per axle.
Also, while I could see separating classes by weapon type for the reasons you provide, I don't see how it makes sense to allow someone to continue with the class with their weapon but handicap them by preventing them from operating their firearm as they normally would. That is, how does keeping a student from using moon clips while still allowing them to use their revolver simplify the instructor's job in any way?
In your example, for instance, it might make sense to keep the person from taking the class with their semi, but it wouldn't make sense to let them take the class with their semi but then prevent them from ever turning left during the class.
Finally, if the instructor had a constructive reason for the limitation, that reason should have been provided promptly when the question was asked.