Was protocol being followed?

While I was away at college my roommate asked to smoke (cannabis) in the room. I agreed and about 30 minutes after smoking we hear the loud banging of cops at our door. I didn't give the cops permission to come in, my roommate did by opening the door. They saw a makeshift pipe on my desk and threatened the dogs if I didn't give up what I had. I gave it up and was arrested. My roommate faced no arrest or even sanctions from the school. The whole situation is really shady because I was under the impression that everyone involved with a federally illegal substance should be detained.