Consider the following scenario:
Player A back-blocking Player B hard enough to cause B to fall forward should result in a major to Player A. It can also easily result in Player B back-blocking Player C.
In most other contact sports, the original penalty would've stopped play. Play being stopped Player B can stop worrying about penalties, position or advantage and can safely take a knee or make a controlled fall, perhaps even use her hands to steady both herself and Player C together. In derby though, play continues. Now there are two possibilities. One, a referee without discretion must disregard any circumstances and call things exactly as the rules state, which means player B could get a back-blocking penalty, possibly a major. Option two is a referee who considers the circumstances, sees that it was C's teammate A who caused B to back-block her and makes the call on A, the initial penalty, only, since it was not B's intent or fault that she back-blocked C, it was A's fault. It is these sort of dynamic and shifting situations and circumstances that demand the presence of, and proper application of, discretion in roller derby refereeing. Any referee who argues that they need not ever apply discretion, any skater that shudders at the idea of discretion, anybody who thinks that a memorization of the rules is enough for a referee should consider this point.