"Why would people facing foreclosure say No to a short sale?"
1) If they are REI knowledgeable, they'll know you're talking pre-foreclosure, not a true short sale. Maybe that leaves a bad impression, maybe it doesn't. It may also send a signal that you're lacking in other areas...and might end up with the wrong deal.
2) If they bought the house to live in, and not to invest in, they might not see the 'advantages' you're speaking of; there may be other reasons, such as a medical/family issue. Chances are, if they are in this category, they also do not know the difference between a shortsale and a pre-foreclosure...
3) No 1099 for the deficiency. This is where knowing the difference between the two is CRITICAL in my opinion. Can you imagine arranging a deal for someone in the beginning, telling them this, then, the deal closes, and they get it anyway? Get your reputation sunk by something like this, and you may have to find elsewhere to do your business. Remember, as we get better, we'll deal more often with OTHER REI's...and word gets around.
4) Exposure: If you deal with a pro, and they detect you don't know the difference, don't expect them to structure the deal to accommodate your lack of knowledge. Its not a rip off, its about each of us being responsible and knowing what our goals are. Thats not the job of the woman across the table from you.
Those were my two cents.