Revealing mistake: During the shootout on the Jet, the second hijacker hides behind a wall and Harry shoots through the wall, killing him. When the bullets supposedly go through the wall, one of them puffs out black powder and marks the wall, showing it's really an explosive charge going off and not a bullet.
Revealing mistake: A metal ramp is visible on the front bumper of the 1972 Ford Galaxie sedan when it rams the motorcycle head on.
Revealing mistake: At the very end of the film Clint's corrupt boss drives away from the docks. He does this at a certain speed. Just before the time bomb goes off the car goes at a much slower pace and also Clint's boss has changed into a dummy.
Revealing mistake: When Clint crashes head-on with the bent Copper on his bike (near the end of the film) It is obvious that a dummy stand-in Cop is used for the 'moment supreme'.
Revealing mistake: When the girl falls out the hotel window after one of the officers shoots her, you can see that the girl is an obvious dummy.
Revealing mistake: Near the beginning, a couple of men are shot in the back of a car. The guy in the middle, Ricca, is supposedly dead but takes several normal breaths as the shot lingers.
Revealing mistake: When Harry is thrown off the hood of Palancio's car, the windshield runs into the arm of a crane, killing Palancio. At this point, the arm misses the driver, and the actor playing Palancio has been replaced by a dummy.
Revealing mistake: Before the pimp is shot dead by the police officer, a circular discoloration is visible on the actor's neck, exactly where he is about to be shot.
Revealing mistake: Multiple times during the final chase, tire marks on roadways are visible where multiple takes of the stunts happened.
Revealing mistake: After the pool massacre, a police cop walks away from the scene and we see the pool with dead bodies. If you look carefully, you can see a "dead" woman in a red bikini moving her arms as if she was treading water.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).