I was at the G20 protests. They only rolled this stuff out for groups that didn't have permits or unlawful gatherings of people obviously looking to enjoy a good mob. When the actual marches went through downtown, the cops stood by and let them march through the city and do their speeches.

Lots of people probably don't realize that Pittsburgh had two "riots" during the last two superbowl wins where the cops were utterly outmatched by the number of people in the streets. I personally got to crowd surf off flipped cars and people were setting fires in the streets for shits and giggles. The mayor clearly didn't want the same thing to happen during the G20, so lots of cops came in from out of state, and they basically acted as temporary security guards while the most powerful people in the world came to hang out. Perfectly understandable response to a situation where it was obvious that people were going to protest and try to riot if possible.

At one point I was waiting on Liberty Ave for the protests to come down toward the Strip District. There was a line of over a hundred police shipped to the intersection by buses, all armed with different types of non-lethal crowd control weapons, with an LRAD in the centre. The protesters knocked a dumpster down the hill toward the cops, which hit some cars on the way, and some folks stood a few hundred feet away singing protest songs and obviously not sure what to do. The rest of the group went back up through north oakland where the cops surrounded them, sounded the LRADs, and used tear gas to disperse people. The LRADs were mostly used for their dispersal announcements, they used a lot more tear gas to break the group up into sections and get them to scatter. It was surreal to be in the middle of, but when they cops got toward me, I just motioned toward my camera and let them know I was documenting stuff and not there to protest, they were all nice to me, and many of them took time out to chat and pose for photos when things weren't dicey.

tl;dr: Those guys in riot gear are people too, they were there to keep the vast majority of Pittsburghers and the G20 delegates safe during an unusual political event. It's hilarious to see it called martial law when a few blocks away people were just heading to the bars like any other day. People should recognize that if they want to get a mob together when the President is in town, they're going to have a bad time, and it might involve sound cannons.