I'm sorry for what happened to you, but at least it was only 15 bucks and you are safe. 15 bucks is way less than any time it happened to me. These guys have probably already taken a few thousands from me.
I live in Sao Paulo, Brazil and get robbed at gunpoint every 6 months or so. Some things people said in this thread, like not hanging out on the streets at certain times, learning self-defence, etc. just doesn't apply here. I'm thinking about getting a gun myself for a long time now but, guess what, it's impossible for me to get it by legal means, which mean, I'm seriously thinking about going to the other side of the law myself. I'm just a computer programmer. I study, I work five days a week, I watch movies, I play video-games, I use Linux. I'm a (almost) normal guy. I just want to live my life safely but I don't have this option unless I:
a) Get rich and buy a nice, very safe apartment, bullet-proof car and all kinds of expensive security stuff. (A lot of people do that here, many adapted bullet-proof cars on the streets, seriously!)
b) Start walking around with a gun and become a criminal too for doing so. The badder the safer is the rule.
Mobilizing the population, protesting against the government, anything like that can get you in jail quickly. Shooting people around will hardly ever get you behind bars. That's my city.
Sorry, this is getting too long. The bottom line is:
I never got robbed in a year I lived in America. I just got robbed at gunpoint again in less than two weeks I'm back to Brazil. Guns are not allowed here.