Sure, there is a open drug scene in parks in Berlin, but there is no such thing in Stuttgart or Munich. Historically the germans travelled to Holland to buy their drugs and it's still the main drug distribution hub in europe.

It's the main distribution hub, but for usage as long as you stay away from Amsterdam it is really not bad. It's also the primary Ecstasy manufacturer for the EU, this is just a trade thing.

I struggle to understand how come Dutch people allow such levels of drug production in ther soil.

What is there to understand? Drugs = money

Such a small country full of illegal production points. I guess there is sufficient data to find them, I am sure others with geospatial experience share this thought.

One point is that Rotterdam is one the major ports in the world, also the wealth has always come from internal trade. Great infrastructure, near to the bigger consumer countries and also not so densly populated rural areas where you can hide labs.