From reading an existing question, it can be illegal to take belongings left in the street, depending on the area of the world you are referring to.
So let me present a specific scenario, in the UK, to see how legal it is.
Bob needs metal. Bob also wants to be eco friendly, so he decides a good way to get metal that can be recycled is through soda and food cans that have been thrown away in garbage bins. Bob decides to get his metal from both public bins, that anyone can use to dispose of rubbish in, and through large bins that are used by venues.
Bob asks the venues if he may take their scrap tin cans, but he does not for the public bins he scavenges from. Is Bob at fault for theft in either of these cases?