The basic difference is that if I saw a springer on the floor - thinking it might be a $150 to $200+ GBB and picked it up to take it back to the safe zone - I would be motivated by kindness.
If I stopped to look at something odd because I was confused I am not acting in kindness.
"Tricking" someone into being kind then shooting them is dishonourable in my book. If its just some worthless crap its fine but not a 'broken springer painted black'. If i lost a bottle of bb's and someone returned them to me I would be pleased so that's basically the same thing.
I guess the fundamental difference in our opinion is that you appear think they want to take the potential gbb as a 'prize' (you use the term 'reward') whereas I would expect people to pick it up as an act of kindness. There is no 'reward' in me picking it up its just a nice thing to do.
I have no problems with trickery and misdirection. Its taking advantage of a kind act that's the problem.
Look at it this way: If it was real life I would never compromise my position to return a discarded pistol to someone. But its not real life, its a game, so I would.