Around where I live there's definitely another factor to consider: Opportunity theft. A lot of bikes get stolen simply because it's very easy to do so, especially downtown. On the weekends when there are lots of drunk people around, a _LOT_ of bikes get stolen simply because people are too lazy to walk or they just find it funny to steal a bike. These bikes sure as hell don't end up at some kind of black market, instead they are simply left at some place and somebody else picks them up. It's like a never-ending cycle of bike theft. Also it doesn't help that most bike locks are ridiculously easy to break.
Another thing that I've noticed (I'm an avid freeride-biker) is that there are actually gangs that target high-end bikes. They have spies at known bike-trails and follow people home. If they see an expensive bike they break into cellars, garages and sometimes even people's apartments just to steal a bike. The bikes are then taken apart and most components are sold right away (a second hand fork in good shape can still go for $1000+ in some cases) and the frame itself is shipped abroad and sold separately. I'm actually very paranoid because it's happened to two friends already... :/
Another thing that I've noticed (I'm an avid freeride-biker) is that there are actually gangs that target high-end bikes. They have spies at known bike-trails and follow people home. If they see an expensive bike they break into cellars, garages and sometimes even people's apartments just to steal a bike. The bikes are then taken apart and most components are sold right away (a second hand fork in good shape can still go for $1000+ in some cases) and the frame itself is shipped abroad and sold separately. I'm actually very paranoid because it's happened to two friends already... :/