I wouldn't be surprised if a local professional photographer was already given an exclusive contract to sell photos.
An agreement with a photographer does not bind third parties. Unless you required third parties entering the property to assent to an explicit agreement not to photograph, or an agreement restricting the usage of photographs taken, then they can use the photos they took and own rights to in any lawful manner once they leave.
I doubt that vendors can just attend an event and sell something like hot dogs for instance without some kind of agreement.
In most places, it is probably not lawful as a business/food vendor to go onto someone else's property without permission and conduct sales on-site; you can lose your license that way, also.
On the other hand, there is no restriction on taking pictures anywhere you are lawfully present.
Although, in some municipalities, taking pictures of certain public objects (Such as the Hollywood sign in California) for a commercial purpose requires a permit issued by the local authority, but that is a matter of special local law used to help fund the maintenance of those public works.