true it would be a problem to outright ban backpacks and large bags. During the Olympics there were a very large amount of police and security at venues and on the street, if you had a backpack or bag you may have to submit to a extra line of scrutiny than people with nothing.
also in reality the organizers of a gathering could reserve the right to refuse entry to any one carrying backpacks or what ever they wish, for the safety of there guests.