Plot hole: Towards the end the Rich Purgers call for reinforcements, and Shane, Leo, Eva and Cali are hiding behind a wall. A guy somehow opens a door in front of them (without being seen by the three facing him), exposes himself and shoots only Shane, numerous times, to cause his death. He should have been spotted by the people looking in his direction. Also, if he was that well hidden he could have simply sprayed the entire group. He unloaded 5 or 6 shots to Shane's chest...when he could have taken them all out.
Continuity mistake: At the end of the movie after sergeant has been shot, they are rushing him to the hospital. Watch the headrest of the passenger seat. It is in place, and then when he is getting in the car, the headrest is gone. Then, it is back in place again.
Chosen answer: "They", which was the Government, targeted low income buildings and poor neighborhoods. The Purge was being used as cover in order to eliminate people on welfare/getting assistance in order to save money and to increase the standard of living for the wealthy.