Why did UL allow this confusing 3 wire Fire Sounder from Edwards on the market

When I started working on fire alarm systems in the late 70's one of the first panels I worked on was an Edwards (EST Systems) 8500 which utilized a very confusing 3 wire set up on on there bells and horns , which could be used on AC and DC sounding circuits . While all other fire alarm bells and Horns had 4 wires  to allow for proper signal of a single wire break causing a trouble condition at the panel. The Edwards set up could allow for the bells and horns to be disconnected and not known till there was a problem as the white wire could snap off the bell   as I had seen happen at a couple different sites.

To make matters worse they had a confusing  red, black and white wiring set up white was negative and red and black wires where positive very confusing all other devices black is always negative. With only a very small tag on back of sounding device to show how to wire it properly and even it was confusing unless you where properly trained on how circuits where wired. You where suppose to read the install manual and follow it but many installers did not have a copy and none was available  on site and this was before you could just go on internet and find one  and caused nothing but headaches in the field as Edwards did not give out manuals unless you where an authorized installer . They kept this confusing  wiring set up well into the late 80's  till Strobe lights became standard on fire systems. Its was so confusing I actually saw Fire system  drawings  for the Investment building in Pittsburgh  with the drawings showing 3 wires coming from fire panel for each horn circuit  when only 2 where used and who ever designed  the drawing  had no idea they showed the horns wrong and local fire official  signed off on these bad plans as well.  Well thank god they finally did away with the 3 wire bells and horns but there are still plenty of them out there causing confusion for any one attempting to service any of these older systems or convert them to newer systems.

 Today most horns and strobes are only for DC only or AC only  circuits and the  few multi voltage fire sounders available  are easy to wire and no confusing wiring diagrams as horn automatically converts to what voltage is present .
This auto conversion circuit was not an available option back in 70's and you would have to stock separate AC and DC bells and horns etc and this is probably why Edwards came up with and was allowed to use this 3 wire circuit on there devices as they had panels which used DC  voltage sensing circuits to allow smoke detectors to be hooked to it  , but had AC powered horns I took such a panel out years ago in a school in Hampton Township.  The panel had no back up batteries and where intended to be hooked to a generator for back up power.

 I like Edwards Industrial line of signaling products- bells, buzzers, horns, notification lights etc have them in many of my customers factory's .  I have used them for over 35 years but this was one case they could have done a better job to prevent problems .