The scam works purely on the law of averages.
If you ask one hundred people to give you a pound, and ninety-nine of the refuse, and one agrees, then you may consider that to be acceptable odds.
If you extrapolate the logic of that concept and increase the 'ask' to an indeterminate amount (i.e. how ever much you can screw out of a stranger's bank account) and up the ask to hundreds of millions of people - and remember you only need one to make it workwhile - then the chances of finding someone who will play along are more than good enough to make it worth a punt.
The sad thing is, individuals who would never consider themselves naiive or stupid in any way have been taken in by just such scams as these, and remember, you only need one - any more is a big fat bonus.
It is a brilliant idea, relying on the inherent goodness of a large part of the world's population overcoming their natural suspision of something being too good to be true.
BTW, my good friend Mr Kodogo is suffering from a cash transer empasse at the moment, so do mail me for some details, and I'll cut you in.