I am guessing having the brokers may add somewhat (along with many other things) to demand for highly demanded times, e.g., the owner who goes through a broker to rent may not otherwise use the points, or would use them at a time other than Oct to Dec or race weekends that would likely attact more renters. However, I look at the legtimate brokers as a service that is very good to have as an owner because many owners may otherwise relinguish points that they cannot use since they do not want to try doing a rental by themselves, and any owner always has some risk of something happening that would prevent being able to use some points in a given year or two. (I have never personally rented points.) Moreover, having brokers gives one a market to look at to see how much their points are actually worth in the rental market and a market to send friends and other acquaintances to who may want to rent (which I have done five times, and, interestingly, none of those five wanted either high demand times or high demand rooms).
Renting is expressly allowed in the official documents and there is serious question Disney could legally prohibit it even if that language was not in the official documents. Not allowed is for the member to engage in renting for "commercial purposes," a vague term that Disney defined somewhat by guidelines which state that a member will be presumed to be acting for commercial purposes if he makes 20 or more reservations in any given year (a rolling year so any 12 month period). The members who are using brokers are typically not each doing a lot of reservations per year, and the broker is acting not as an owner subject to the commercial purposes rule but as an outside agency and middleman providing a service to owners. Thus, Disney lacks a legal basis to be able to prohibit brokers from operating.