1) The police are still supposed to arrest people who are caught with cannabis (It's a class B drug - in the same group as codeine, ketamine, and ‘spice’ - whatever the hell that is).
In reality they typically let people off with a warning but it's still a drain on our already hamstrung police force
2) Drug dealers who sell weed tend not to declare and pay tax on their earnings. Depriving UK Gov of some seriously needed income per this article's headline.
3) Drug dealers tend to sell other drugs, usually harder drugs. Thus exposing people who want to get stoned to a plethora of other drugs that they might otherwise not have come in to contact with.
4) Those people who are hooked on weed (or any other non-prescribed drug) are criminals...and cannot seek help from anybody including their doctor. This keeps them trapped, and over a long enough period of time will do mental and potentially physical damage to them that our NHS will incur costs to fix. And when I say NHS, I mean the tax payers, and when I say tax payers - I mean me.
All of that is to say - fuck UK drug policy. I remain hopeful that it will change though...