A crowd waits in Union Station, and a few people in yellow POLICE vests keep ordering everyone else to move along because the area is closed. The speaker watches this with dry, tired irritation. The opening lines, "a bunch of people / standing around / doing nothing," sound flat and casual, and that applies to the vested people as much as the rest. Nobody seems to be doing anything useful.
The turn comes at the end. The speaker says the officers' job is "to make life / just that much harder / for the unvested." The made-up word "unvested" is the sharpest touch. It reduces the whole situation to who has a vest and who doesn't, as if authority were nothing more than a piece of clothing. The humor is sarcastic, but there's real frustration under it: the speaker senses that rules can feel arbitrary when you're the one being herded around.
The mood is deadpan and a little weary, with small everyday annoyance standing in for a bigger point about power.