Other mistake: When Ricca is pushing his way through the crowd, just after a man yells "Ricca, you're a killer" a shot of the crowd shows a sign saying "UP YOUR RICCA" which makes no sense. Surely this sign was meant to say "UP YOURS RICCA." (00:03:50)
Other mistake: The vigilante bike-cop slaps a supressor on his Colt Python. A suppressor on a revolver doesn't work without serious modification. There's only one revolver I know of which would be able to use a suppressor, namely the Russian Nagant M1895. And that's due to the gas-seal system in the cylinder, which seals all the out-gassing, only letting the gases out the muzzle-opening. (01:01:00 - 01:10:00)
Other mistake: In the end credits, Callahan is misspelled as "Calahan". (01:56:50)
Other mistake: After the pool massacre, when the shooting stops, at the left of the screen and right off the policeman's helmet, a man sits down and then lifts the beach chair up.
Other mistake: When Callahan meets Sunny in his apartment lobby and she asks him, "What does a girl have to do to go to bed with you?" Callahan replies, "Try knocking on the door." and if you look behind Callahan at the Trolley going down hill, you can see a kid waving to the camera.
Other mistake: When Harry and Early Smith are in Lt. Briggs' office, Briggs' first name on the desk nameplate is spelled "Neil", but when Harry and Early Smith leave the office, Briggs' first name on his office door is spelled "Neill."
Other mistake: Before the shooting, we saw a reflection of the officer in the outside mirror of the limo. The glass is rectangular. The actual mirror of the '73 Cadillac Model 75 had rounded corners.
Other mistake: In the final credits, DiGiorgio is misspelled as "DiGorgio."
Other mistake: In the scene where the mob boss gets interviewed in the garden atrium after being freed, the news media pushes up to get really close to the podium. Note that the first reporter, dressed like Gilligan, trips and almost falls into the podium. He catches himself but has to keep his head down during the rest of the shot.
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).