The police force in any country reflects that country, as has been said before the police don't make the laws, they are paid to enforce the laws the politicians make on behalf of the citizens. No point in blaming the police, they don't particularly want to be chasing people down for personal use of relatively harmless drugs but it's what the country wants. How do we know it's what the country wants? because they voted in the politicians that brought these laws in! You want marijuana legalised? vote in the politicians that will do it for you, don't blame the police.
On the other hand it doesn't matter to the police whether some drugs, all drugs or no drugs are legalised because they are the ones that have to pick up the pieces after people have used substances including alcohol. They are the ones that have to tell relatives their loved one's dead because they were hit by a driver under the influence of a drug or drink, they are the ones who have to get the bodies out of the vehicles, the ones who have to deal with the bodies and families of those who OD. They also deal with the violence that erupts because of people taking things they can't handle, they deal with all the nasty things associated with drugs and alcohol, never ever the 'nice' things that people say substances like marijuana can give you, no for them it's the blood, snot, vomit, deaths and the battered, the bereaved, the helpless and the lost. Legalise everthing and they will still have to deal with all this.