There's something fishy going on here. Those Cops sound like they're after him.
I wouldn't think after him so much as confused about how much power the city has to restrict things.
For years the Port Authority in Portland declared it was illegal to carry a firearm into any port. Problem is the state actually has pretty clear definitions on where and when it is legal and illegal to carry a firearm, and the Port Authority over stepped it's authority in this matter. Yet the law backed them for years.
The same thing happened in Sea Tac, they never officially declared it illegal, but many believe you cannot carry a firearm onto the Seattle airport. While you cannot carry a firearm into an unsecured area, if you can carry a firearm into unsecured areas of airports is entirely up to the state you are living in and how the laws are set up in those states. Traditional open carry states (states that have protections in their constitutions) that set up bans in this nature soon find out that the ban is unconstitutional. It's easier not to carry a firearm into the airport then to try and fight an illegal law so nobody does, but that doesn't make the law anymore valid.
The same things happening now in ports, and other similar areas was already covered once before with the passing of the Firearms Owners Protection Act in 1986. It was discovered that the BATF, was far and away abusing powers given to them and well over 75% of the prosecutions for firearms they had made where unconstitutional. This was mostly at a time when automatic firearms where banned, and the some 175,000 preban automatic weapons where closely watched by the BATF, who used their wide ranging powers to harass and illegally confiscate those weapons in an attempt to completely remove them from the public. Even through not a single one of the 175,000 legally obtained and licensed firearms was ever used to commit an illegal act.
I bring up that act specifically because bands of firearms in unsecured airport areas (especially parking lots) goes completely against the Safe Passage section of that act. Yet they are willing to ignore what is already established federal law with what would essentially be a restrictions imposed by an agency of the goverment (the TSA controls security in airports).
I have had difficult dealings with the police on three occasions due to firearms. Once being actually arrested, and nearly having my vehicle impounded because a sheriffs deputy couldn't let me show him my carry permit. Once for not telling the officer who stopped me that I had a firearm; which is not required, and frankly not a great idea, how would you react if someone you just stopped for a traffic infraction suddenly told you they had a firearm, how are all the cop shootings at traffic stops not supposed to spring to mind? The last time was for having a pistol in the parking lot of a bank, again not illegal, carrying the firearm into the bank would have been, but the parking lot is a different story. Besides the officer only saw the firearm because I removed it from it's holster to secure in the truck once I got to the bank, I could have walked right into the bank and nobody would have noticed a thing.
All that said I think it likely that they have a local ban on firearms in the city hall that is an illegal law under the Minnesota constitution, or state laws, but never having lived in that state I couldn't be sure. I doubt a person that wrote a book on the firearm laws of the state would make such a mistake through.