This confused the hell outta out of me too but I think I figured it out. I'm pretty sure this only applies to the first couple levels, I noticed I was turning on the code then dying but once I got around to level three or four it stayed on.
Just stick with turning the code back on if you need it but hopefully it'll stay on like I mentioned once you get past the first couple levels.
(P.S. That Helicopter area on hard is an asshole!)
Edit: Hmm playing through hard mode again the codes are working now
Last edited by frazzleberry; 02-23-2012 at 12:20 AM.