Your on the money Nearlythere. Ignorance plays a big part. As an erstwhile electrical contractor I often received instructions from developers, builders, architects etc to install a fire alarm system as a last minute addition to their new-build or refurbishment project. Often the imperative came from building control when they identified that the omission of a fire alarm was unacceptable. So the design, construction, inspection, testing and commissioning of the fire alarm was dumped on me, the electrical contractor. In those early pre-enlightened days I went to the wholesalers who put me in touch with the Rep from JSB or Tann-Sychronome and he told what I needed to do which included MCPs at every final exit!
I often see drawings now marked "fire alarm to comply with Bs5839......" but without the various elements of the system being indicated on the plan. It seems to get the plans approved by BC but leaves the system design to the installer who will probably be as unenlightened as I was in my pre-enlightened days!