People just get up, grab their stuff and leave. How often do you pull out your drivers license to show id? Maybe once a month? Most people probably realize when they are looking for it that it’s gone and may not recall where it is a few weeks later. At least with phone number and email, the theater can contact them that day or the next—a lot quicker and easier than writing a letter to the address on the drivers license that takes a week or so.
It must have happened enough times to institute a new process.
