They were in an incident and were deciding what charges to lay.
It's a tricky thing laying charges.
Laying charges is not abuse of power.
Now - if the cops were inventing or fabricating evidence - saying the person did something he did not do - then this would be definitely abuse, but it's not clear that happened.
But if someone standing in the street with a gun is doing something a cop can feasibly consider 'public nuisance' - and it's within their right to charge someone with 'public nuisance' - then the issue here is not the cops, it's the loose definition of 'public nuisance' and the cops ability to arbitrarily apply it.
The article is mostly quotes from the ACLU, so it's only one side of the story and selected information, it would be interesting to know more.