This slightly different use case has probably been discussed here before, but I think it also belongs in this thread.
I struggle with what it means to own private keys. I think we just "know" the private keys. Anyone who knows the private key has complete access to the associated bitcoin. So if I try some obscure brainwallet phrase and discover a long unattended address with BTC, is this the same kind of theft as the OP's fraud use case?
Since we can't communicate a warning shot to the person who set up a bad brainwallet passphrase, how can any be sure that the private keys have not been lost due to bad memory/security proceedures? That might be the same thing as finding a $20 bill in the middle of Central Park in NYC. Untraceable owner and no confidence that the authorities can get the money to the rightful owner. Most people would treat it as a windfall, and no one would consider it theft. I think this last point depends on the amount of money found -- if you found a satchel of $100,000 cash, taking it to the authorities would make sense.
But back to the question of "knowing" vs. "owning" private keys. Can there be clear delineation of how to interpret this?
I am not sure how exactly to define how someone can "own" a private key. I would however say that it would generally be considered to be 'stealing" bitcoin in the event that you use some kind of exploit to learn what someone else's private key is
Well, at least it is a "moral question". As someone said above, discovering someone else's private key would be more or less like
finding someone else's money:
There's the possibility that the rightfull owner had
lost (does not know anymore) that private key (in the case of the found money, it was a "lost money"), and as a consequence the bitcoins would have been lost anyway: in this situation, as it would be "impossible" to find the owner, I believe "recovering" the coins to the ownership of the "person finder" is acceptable.
But there's also the possibility that the "original rightful owner" still has knowledge of the private key (maybe it's someone else's cold storage). In this case, it would be totally wrong to steal the bitcoins from that "found" address. (for the example of the found money, it could be a "hidden treasure").
As it can be very hard to discern the situation (if it was lost or hidden money) the person who eventually collides with someone else's address will be in a moral dillema situation.... But, anyway, key collisions are virtually impossible to happen, fortunately...