Unless the lease with the County says the lessee CAN'T prohibit firearms, the same rules apply as if the promoter actually owned the property. By leasing it he gets all the rights the actual property owner has except the right to sell it. Most leases even hold the lessee responsible for taxes, maintenance, and repairs (the "triple net" clause on most leases)
Ask yourself if the individual being escorted off the premises had not "flashed" the weapon, or had been carrying concealed, would have even been bothered.
This has been pretty much answered time and time again but I thought I'd answer anyway. The typing helps with the arthritis in my fingers and hands