I suppose this stern attitude (in the US) to some private person (a neighbour, a plumber or cleaning lady calling in for a day's work or something, noticing an object that they realize might incriminate the guy living in the house etc) taking care of that object and handing it to the police, that route making the evidence invalid even if it does in fact provide something forensically important, only applies when the object was gathered on the suspect person's property, in their house, their garden, their car etc? The point about an attendant charge for burglary is easy to see (and would apply in many countries), but it sounds strange if even the folllowing example should be true:
My neighbour says his wife is with her aunt in another town for a while. A week later he says she has fallen ill and has gone to an unspecified health spa with auntie and will remain there for a while - indefinitely. Two days later, near a shopping mall on the outskirts of town, I notice him walking around with a sports bag with something bulky in it. I silently follow him at a distance and see him dropping the bag in a massive iron waste container (which will be driven to the waste recycling plant and emptied every few days). Five minutes after he has left, I walk up to the container, climb up on a crate and look in. The container is half full, but I manage to spot the bag, reach down and haul it out using gloves. In my car I open it, still with gloves on, and it turns out to contain some empty packages of strong sleeping pills and ladies' clothes smeared with darkened blood and grime. I promptly take the bag to the police, explain how I got hold of it, who dropped it and why I think it's important. They inform me it might be unusable/inadmissible as evidence in a police inquiry or a trial, because it was retrieved and handed in by me, not by a policeman, and furthermore I might have planted those incriminating objects into the bag, who knows? So even if it should turn out that it *is* his wife's blood on the clothing and both the bag and the clothing have my neighbour's fingerprints and a few traces of his hair clearly on them, it could be argued in a trial that I have helped my neighbour or even killed someone myself: his wife or somebody else.
Or the police saying: this isn't enough to motivate a search warrant at his home or a query for his wife, though it could have been if it had been a cop taking care of the bag just like you did.
This sounds bizarre as a ready-made response to a neighbour or somebody else who has been in touch with person X secretly getting hold of something well away from X's home and property - something they think X has been handling during a crime - and offering it to the police.