Answer:
The laws vary from state to state in regards to dumpsters in public areas,In some states the contents of the dumpster are considered the property of the waste facilitiy that owns and supplies the dumpster and it is considered theft to remove any items from that dumpster.
In some states if the dumpster is enclosed behind a fence and has a gate you can not enter that area to reach the dumpster.In all 50 states you can not tresspass on private property to search through a dumpster or trash cans that are on said property,such as if a homeowner is remodling there house and has a dumpster there for construction refuse..
There are also county/city/town ordinances that can make dumpster diving illegal even if state law permits it.
The vast majority of the time dumpster diving is very far down on the list of laws to enforce and mostly goes unnoticed..