The police are called and complained to by another tax paying citizen. The police show up to check on the complaint. You answer the door, they tell you why they are there. Then they ask to see ID's, you allow them to come inside and look around. They check id's on all subjects they see, they issue tickets for mip, and what ever else. They leave, you hope they do not return later on.
The police are doing there job. Now you can argue that they could not see in the house all you want. But when you open that door, they can clearly see in the house or building. All they need is to see one person drinking they believe is underage. Yourself being intoxicated answering the door. If your age is questionable that gives them reason to check id's.