Bingo. You are totally and 100% legitimately allowed to hijack, block, and/or de-auth rogue WiFi access points broadcasting the same SSID as you so long as the blocking is targeted at your own property (some leakage is inevitable).
If attendees were naming their personal hotspots "SmartCity" then SmartCity would have a point. They weren't.
Uh, no. Just because you think this should be the law doesn't mean it is the law.
Instead the law in the United States is that you aren't allowed to jam anyone, aren't allowed to interfere with the operation of other computers on the internet, and whether the computers are on your real estate or not has nothing at all to do with it.
In the first paragraph you mention property, but SSIDs aren't property. They don't own "SmartCity" and you don't own "RandomPundit" and I don't own "DidMyResearch" either. The legal point seems to be whether Smart City's actions were limited to the physical space they owned or leased, not what SSIDs were involved.
Yeah, I'm not seeing any evidence that they were the first to be using a particular SSID when they shut them down.
What happens if we both have the same SSID and move into the same area? Who 'owns' that SSID? Smart City gets to just roll into town taking over any SSID it wants?
As far as I can tell, strongest signal wins (assuming it's within transmit-power limits). Probably just as well, since adjudicating disputes over who owns what SSID in what space would be a total nightmare.
If attendees were naming their personal hotspots "SmartCity" then SmartCity would have a point. They weren't.