I would guess the method of braking assist is the reason for the exclusion. All gas engine vehicles use a vacuum boost to reduce the braking effort. While towing this vacuum is not available and braking requires more effort, but with more effort the brakes can be applied.
On the popular pickup diesels a hydro boost system is used - the power steering pump supplies hydraulic pressure to the brake master cylinder to aid with brake application. With the engine off the effort to apply the brakes is much greater than vacuum assisted systems.
