Because cops DO have that honor code among them and will always have it.
It's only disgusting to us now because we're being pummeled with news reports of trigger-happy cops.
Even with police corruption, I bet most cops will look the other way and not rat out their fellow cops.

I always got the impression that Foley was referring to the situation of 2 cops coming to blows. Macho thinking but it came across, however dubious, as 'if my fellow officer hits me we'll settle this like men'.
Considering the lengths Foley goes to get Maitland I'm sure if he knew a fellow cop was on the take his reaction would be different.
Besides going by perception and stereotypes in a place like Detroit it wouldn't surprise me if Foley's belief wasn't something real Detroit cops uttered.