Lots of places in Europe allow it in restricted areas. There is a children park in the middle of Paris known for being almost exclusively used by drug addicts and dealers.
They know the drugs will be sold anyways so the logic is to restrain it to a specific place. In Berlin one of them is Gorlitzer Park, the police is circling the park all the time to avoid crimes (other than selling drugs)
It's not really a problem these days since you can order all kind of drugs (anything from weed to coke) from signal/telegram groups and get them delivered to your door, sometimes faster than food deliveries