It stays for the reasons I outline above - the fact that it isn't actually part of the highscores any longer, and as such I was too lazy to go in and remove it manually ahead of time.
As for how the exploit worked - let's just say that it involved a last minute suggestion during the Beta (I'm looking at
you, aXidal!) which I didn't bug test properly. Attempting to buy the land mines twice resulted in being able to use them more than once, wrapping the ammo count around from 0 to 255.