Its a body count thing....
There's been a chronic shortage of skilled people in the UK for some time now -- there's tons of people who can Powerpoint up a storm and the like but comparatively few that can do complex engineering tasks. We've covered the shortage for many years by immigration but this still leaves government agencies and contractors at a disadvantage because they have to hire citizens (although that's been relaxed to permanent residents in some cases). Adding further barriers to entry like the Reagan era War on Drugs "ban all pot smokers" further shrinks the pool until you're left with very slim pickings to work with. Relaxing the rules somewhat is just a nod to reality.
I personally have never willingly hired on to a company that drug tests. I've had to do it once and it was a pretty gross experience (I don't produce urine on demand....). Its also a gesture of submission to the whole Nixon/Regan "War on Drugs" which really was a war on people, especially minorities. I've had to live a pure and blameless life in the US but that's because as an immigrant I can end up in a world of hurt with a drug conviction -- the INS (as it then was) regards drug offenses as 'crimes of moral turpitude' (their wording, not mine). Now I'm a citizen I don't have to worry about the Migra and as I live in California the stuff's to be had over the counter (with a senior discount, of course). Its all very ordinary, in fact you tend to wonder what all the fuss was about...it certainly wasn't about 'drugs'.
Biting the hand that feeds IT
