"Why should he not be allowed to get a non-related issue fixed?
That is the reason why he would want to get it replaced now. Just in case something goes wrong."
"Just in case" isn't a reason for exchange. Let me and Apple know when an issue actually arises and then let's have the discussion on the policy.
Here is the problem with that. One of my best friends has had his iPhone 3G for only a few months and was having serious problems with dust under the screen which I guess apparently was a known issue. He took the phone in and the tech found that the water sensor was tripped (due to keeping the phone on the bathroom counter during shower) and refused to look at it and he couldnt get the defect replaced under warranty. Thats BS. Even if a litte water was spilled on it it wouldnt cause the dust under the screen which was even a known issue. Thats why I cant blame the guy for worrying about wanting to replace the phone before something actually goes wrong that isnt related to having water on it. Also i think water sensors are faulty because they should activate only when water gets in it and not because of moisture. Down here in humid Florida I assure you the water sensure can be tripped when the iPhone is exposed to moisture in the air on a heavy humid day, I have heard of it happening.
Now Im not condoning people who break thier iPhone and then EXPECT Apple to replace it. I just think that you are making the issue out to be black and white but there is a lot of gray area to it and it really needs to be evaluated on a case by case basis.