It's called "organized crime".

I've read about other types of muggings - a crowd passes you, someone hits you on your pocket and your wallet flies out of your pocket. A good reason not to put anthing in your pockets.

As for the camera thing - all cameras come with wrist straps or neck bands. Use them... as a deterrent to theft. However if someone does try to steal your camera, well, you may end up getting choked.