Why do many on the left criticize America's involvement in Iran in 1953 and Chile in 1973, but . . .?

also its involvement in Iraq? If "installing dictators" in Iran and Chile was wrong, why wouldn't getting rid of one and replacing it with democracy be right? And if Saddam Hussein was a good counterweight to Iran, then why couldn't the Shah and Pinochet be good counterweights to America's enemies as well? Thanks.