I think that has to do with people perceiving honesty. Someone saying they just want weed and alcohol gets ups for being "honest". There is an automatic skepticism for the "SOB" story beggars.
One, there are places to go if you really need help. If you're starving and you need food and shelter, there are options. If you just want money, you panhandle. People are sick of hearing of pro bums who rake in money by begging. I remember when I was in college in Denver, reading stories of bums making more money in a day than I did working. There are people who get caught driving a Cadillac to their panhandling grounds. Those sorts of stories will sour people to handing out money for that sort of thing.
Yeah, true. At least if you're asking for drug money it does come off a bit more honest. Still though, the guy had a kid laying on his lap and they were in rags. Not one person donated until the end but 15 people give him money to help kill himself?