Not usually politicians putting forward the specifics of the rulings - for the very reason you have stated.
I hate the nanny state too but for once I think this ruling comes from a background of common sense.
As has been pointed out, the ruling regarding lights being required to be fitted in pairs has been removed to accommodate light bars, so there is acknowledgement of their effectiveness and prevalence on roads. The usual knee-jerk approach is to just BAN IT.
The output of these lights is truly brilliant, so the concern with them being fitted to the roof is the level of reflected light from the bonnet, wing mirrors, bullbar and other surfaces forward of the lightbar and within the driver's field of view. Exposure to that reflected light in the foreground does a great job of reducing the sensitivity of the driver's vision to hazards in the background (out in front of the vehicle) - whatever the local wildlife is. Plus you can roll in the effect on the usual issues of driver fatigue.
The issue with mounting them on the top of the bullbar is of course pedestrian safety, the same as fishing rod holders - except there is no other practical place to mount fishing rod holders so there are provisions for them being mounted temporarily.
What this ruling does for lightbar owners is makes these great products legal fitment when they weren't before unless in pairs, provided they are mounted appropriately. Since they still work fine mounted where driving lights usually go, this is a step forward.