My guess is that they are using the lots as a safe lounging area between feeding areas. This years young are still easily susceptible to predators and a wide open parking lot allows the birds to see any potential predators from a long way off [[That's the same reason you typically see geese and other birds in the middle of open fields as opposed to the edges).
Anyone that keeps a boat docked knows the damage that gulls can do. Their excrement can literally eat paint and canvas on a boat, and will do the same thing to the paint on a car if it's not quickly removed. Just look in the background on the video and you can see the mess that the gulls have left on nearby cars. I'm sure they were having some significant problems with these birds if they resorted to using lethal force.
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