You cant even drink your own booze in your room?.
You can, but you cannot have an official event serving alcohol without permission of the hotel. While bringing large amounts of alcohol to, and consuming it in, your hotel room is usually frowned upon by hotels (not just German hotels!) they won't stop you. A room party isn't an issue either, as long as it does not get out of control. However, consuming alcoholic beverages not purchased from the hotel in function spaces such as for example the lobby, hallways, conference rooms or ballrooms requires permission of the hotel which involves a nominal fee.
Basically, a hotel is restaurant with bedrooms. A large part of its revenue comes from selling food and beverages.