Actually, no, it isn't. The "big stall" is designed to allow a wheelchair to have access. It is designed with hand rails to allow a person to transfer and balance. It is designed for room to allow a companion to a disabled person. While I don't have a problem with using it if there isn't a person waiting for it, I would hope anyone using it would be quick and allow a person in a chair to have access first.
I was totally amazed this past summer at the number of people who don't understand why these larger stalls are there. Until my daughter was confined to a wheelchair over the summer and could only use the handicap stalls, I really thought most people did as I did. But having my daughter in a chair this summer really opened my eyes to other people's attitudes.