Two reasons:

1. There are long term plans to legalize it. Currently it may be against International Law so it will take a while until it can be opened for all. The first action will be scientific observed modell citys.
2. The decriminalisation will aim to destroy the black market. This is done by allowing social clubs than can grow their own plants and hand their members up to 50 Gramms. That is a lot and the implicite idea is that this amount will also allow a bit of interchange without being enough to drive a illegal business.