Its a logical and good assumption. Without coming across as being defensive; what would a person using BP/W needs to fuss about when its was already correctly sized? You set it up, you don it an you are ready to go. No further adjustment is needed. Besides, my instructor, due to his former vocation, would have no problem speaking is mind, albeit in a positive manner of delivery.
The fussing example was intended to illustrate the Instructor's perspective on training students in an introductory/Open Water scenario. The Instructor's focus is and should be primarily on the new OW students. If there is a distraction (as determined by the Instructor), he may feel that he needs to correct that distraction before continuing.
To step away from the argument for a second, I honestly don't see how the DMC wearing a BP/W long hose is a distraction...a rebreather, yes, but BP/W, no. So, I agree with the original premise: It's silly to disallow the usage of a specific gear package and configuration. Especially a kit that is as proven and simple as a BP/W/long hose. The DMC is only observing the class. He is not allowed to be demonstrating don/doff, and he still has an inflator hanging over the left shoulder...hopefully the students will emulate the use of said inflator with their buoyancy skills...
A recent discussion with my PADI regional rep was enlightening in that he encouraged me to use different gear configurations including doubles, but excluding rebreathers. As long as standards are met and and that I don't require added standards, I was also encouraged to teach and to include work on buoyancy, horizontal trim, different finning techniques, solid gas management and planning, etc., based on how much time my students and I were willing to commit.
So, the equipment choice and configuration issue boils down to shop policy and each individual Instructor's preferences for conducting their classes.
But, whether the Instructor is biased/prejudiced/ignorant, or shop owner put his foot down about using specific equipment, or the 7 foot hose turned into a snake and bit someone on the a$$, in the end, the Instructor needs to feel comfortable and in control while teaching his class, and the DMC needs to respect that.
My spin in this thread has been, as a DMC, to quietly respect the Instructor's wishes until an appropriate time when a good hearty discussion on his reasoning can take place. A DM will need to assist different Instructors and to adapt to their quirks. Maybe there are some Instructors who the DM will not want to work with. There are certainly some DMs who I don't want to work with!

I teach in my shop owner's pool, and I respect his wishes to use gear similar to the student's gear and that is sold in the shop. The day I own my own training facility, I will wear whatever I please!
When in Open Water, the DMs, DMCs, other Instructor's and I wear the gear that we are comfortable with and which best suits the environment. That gear may or may not be the same as we used in the pool. It depends on the environment and the individual.