I don’t have any issues with the morality of dumpster diving. If someone was to take metal from a dumpster and sell it to a recycler, that would clearly be stealing to me. I consider reuse to be a greater purpose than waste and recycling. The legal issues are much more confusing to me.
Items left on the curb are not considered property, and are fair game. This is based on a case in which it was determined there is no reasonable expectation of privacy once items are placed on the curb. This is why law enforcement can use trash in an investigation and why shredding documents is necessary.
The main concern of dumpster diving is a trespassing charge, since private property is ubiquitous. Here is a post I found useful from a Salt Lake lawyer about trespassing.
A Salt Lake City law enforcement officer informed me that it is possible that any items on private property are considered property regardless of whether or not they are in a dumpster. The legal absurdity of trash being property may be accurate.
Essentially, dumpster diving in Salt Lake City puts an individual at risk of being charged with trespassing, and a much lower risk of being charged with theft. Yet, if an organization was to give some sort of consent, they open themselves up to liability concerns. The wisdom of its not what you do, but how you do it can be applied here. It may not be an easy conversation to have, but obtaining consent and working with organizations during business hours to reduce waste seems to me the most prudent and effective method. This discussion could go directly against the nature of capitalism, and may require change to entrenched process. In many instances those wishing to create something out of waste will be criminalized and forced into the night.