In my opinion if your taking from a roll off full of metal it's stealing, clearly its sole purpose is to be scrapped. But from a residential dumpster etc I view it as more "frowned" upon considering the fact that that material is going to the landfill taking up valuable space egc, so not only are you profiting but your recycling something and giving t a new life to be reused. Your always better off checking in with the owners of course though