I don't think the enforcement is all that selective - any time I took an exam in a large hall, there were professors and TAs pacing around on the lookout. In smaller settings, it might have been just the professor but they all seemed to be trying to enforce the code. Many people may slip through the net but that's different than selective enforcement. If the rules have been communicated, I see no problem having a severe penalty as a deterrent.

Also, you may consider that it's because you're a cheater that you think everyone else is doing it. I made it out of a couple elite colleges with a good GPA and no cheating.