Does a policeman have the right to open a car door without asking?

I was pulled over for not stopping completely at a stop sign. I had 3 pengers in the back. While getting my license, etc. he looked in the back seat, then opened the back door to observe that one of the pengers did not have his seatbelt on. He issued me a citation, but gave the penger a ticket for not wearing his seatbelt. My penger thought the officer had no right to open the door. Any thoughts on this?