Because Usopp is always against great odds. He's the coward everyman who only wins through luck, ingenuity, or by finding his courage at the last second. If Decker's DF cancels out Augur's teleporting, then you're left with a bare-bones sniper vs sniper situation, which is too level-plain for how Oda writes fights. It's not enough that Augur has been built up to be a better marksman than Usopp, he also needs to put the liar in a further disadvantage, which the aimbot DF disables.