Many public events such as sports, concerts, etc have restrictive policies that limit the ability of the audience to take photographs. These policies can vary from an outright ban to a ban on the use of 'professional' equipment. See also this discussion.
So my question is this: what gives the organising companies the right to enforce photographic restrictions on, what is after all, visible to a public audience?
Now I can understand their rights to make restrictions in purely private, by invitation only, events. But when any member of the public can choose to attend do the organisers still have rights to dictate the behaviour of members of the public?