You mean they stole it, WHILE you were riding, and it was right in front of your eyes??
I´ve usually got my DSLR and all optics etc. in the tank bag, worth some 3000 euros, and unless they´ve got a gun or a knife, I damn sure wont give it away to any slimebag without a wrestling match. But it is of course fastened, too, so it can´t be just snatched in an eyeblink.
Sorry to hear about your negative experiences. We got invited to stay with a local family while in Esfahan, they treated us like old friends, and stealing from tourists just felt unheard of (all over Iran really). But sure, it can happen anywhere.