Oh dear.

Whilst I'm not a fan of laws permitting civilians to carry firearms at all (although given the context of the USA, they are somewhat understandable), the law in this situation seems extremely clear. Rosenberg specifically informed the Minnesota police force that he was carrying and was given specific permission to take the weapon to the police station, so it was all cleared ahead of time. Unless he pulled out the gun or started threatening the officer with it, I'm not sure why it became such a huge issue. Rosenberg clearly had an agenda (which is why the meeting was recorded) but that agenda appears to be highlighting corruption, persecution and lack of knowledge by the local officers about the very laws they are trying to enforce.

As another commentator said, Rosenberg committed no crime, only the offence of knowing more about the law than the police, and absolutely nothing riles the police up more than being shown up as idiots (in any country).