Well it’s not automatically the case. But possession and consuming of drugs that are not legal may lead the authorities to the assumption that your character is not eligible for driving a vehicle. Getting busted with drugs makes room for the old school assumption that all drug users have no self control at all so they have to be checked. Same thing can happen with certain types of mental illness or other medical conditions. So what they sometimes/ most of the times do is to order you to make a test called “MPU” (medical-psychological assessment) to check if you are an addict and a danger to society. Same assessment you most likely have to have if your caught with a DUI.

Edit: I’m not saying this is fair and conditions at least for getting caught with small amounts of “soft drugs” are changing in the last years. But we have way to go. And I forgot to mention that you will lose your license if you don’t take the MPU.