He notified the sheriff who is a county official. Perhaps he should have notified the city's public safety commissioner. I am only guessing.
Minneapolis' City Hall is a city building, separate from Hennepin County. I didn't see this in the article, but they may have arrested Rosenburg on a technicality. "persons who carry pistols according to the terms of a [carry] permit issued under section 624.714 and who so notify the sheriff or the commissioner of public safety, as appropriate…"
Since the Minneapolis City Hall is not a county facility, it may have been "appropriate" for Rosenberg to notify the commissioner of public safety."
Regardless, it's a cheap arrest.
Not knowing Minneapolis, this is just a WAG, but perhaps with the size of the city, it is like Indianapolis where the County is encompassed by the city and it is a City/County building with the Sheriff being the lead LEO with overall responsibility. Again,,,just a WAG