The cop had know way to know who was telling the truth. You had a guy with paper work he had know way of knowing what was what. Some paper work with a couple of serial numbers goes a long way.
Those things may be fine for a defense but even the cop recognized there was no signatures on those papers other than the ones by the accused thief. The determination as to whether there is enough evidence to press charges for theft is to be determined by the DA's office. A citizen reported a theft, the cops find the alleged thief with the merchandise claimed to be stolen. What more do you want? Letting the guy go with $20,000 worth of equipment saying that they can sue doesn't do much good if the thief continues on with the equipment and is 5 states away. The guy didn't even have to be arrested, just impound the equipment and make the actual owner produce proof of ownership. If this was construction equipment from a job site, you can bet that's what would have happened. The fact that it was for medical marijuana, the cops just didn't want to deal with it so they just passed the buck and denied a citizen due process.