Jude, why would rich people pick up a diamond necklace in the street and keep it? Dishonest ones would, as would dishonest poor people. The rest would hand it in.
Turning innocent people into criminals? Hardly. Innocent people must mean those who would not commit the crime. Presented with a car with an open door and the keys in the ignition, normal, "innocent people", would not take it. It is no defence, in English law, to say that you would never have committed the crime had not been so easy nor that you were set up by what is called an agent provocateur.