Payment for Airport parking

We picked up friends today at the airport. (Thankfully, we didn't get hassled, blocked or threatened by any taxis, but I was prepared, with a camera, pen & paper, etc. if they'd tried.)
I don't know how long the current parking payment procedures have been in effect, but they were new to me.
You have to pay inside the arrivals area of the terminal, down in a back corner. You then insert the stamped parking stub into the machine, and you're done. Until one of the rent-a-cops explained it all to me, we were condemned to circle the parking lot, trying unsuccessfully to find a way out, like Charlie on the Boston M.T.A.