Yeah, it's the "why" I don't get. Clearly people do cheat, but you are surely only cheating yourself?
There was a lady on that Marathon Investigator site who had cheated Berlin (possibly) 2 or 3 times to qualify for Boston. All her other marathons before and after (including Boston itself) were like 5+ hours, yet she could do Berlin in about 3.40 (with a load of missing timing mats...). Why cheat to BQ when all your other times make the cheat time stand out like a sore thumb?! I couldn't do it, I had a dodgy Garmin result recently which gave me a 5k PB but I declined it from my PBs as I knew it was off and it would bother me not to have earned it.