this isn't news - it's been common knowledge for over a year, and discussed ad nauseum on here - it was only when they reinforced parts of the act a couple of years ago that it became an issue and the reason is simple - lazy legislators, instead of creating a new law to cover precursors (not necessarily poisonous) just tacked it onto the poisons act.
Remember when they banned sodium chlorate weedkiller? that wasn't on the poisons list, but it was because it was a popular ingredient in home made bombs way back in the day.
The precursors list goes back decades but was used more to monitor illegal drug manufacture way back when. Even caffeine powder crept on to the list when it became a popular cutting agent for cocaine.
I remember a bodybuilder a colleague intercepted on his return from a 'smoke and poke' weekend in Amsterdam having his chocolate starfish closely scrutinised as he had taken advantage of the availability of cheap caffeine out there and bought home a kilo of (perfectly 'legitimate') caffeine which he used as part of his muscle bulging diet.