I think the analogy of someone handing you $50 is off - that is a deliberate and purposeful action, whereas dropping some money on the ground or sending it to the wrong (with nothing identifying) account number is not. Seeing a person face-to-face, you can be much more certain right from the get-go that it is going to the right place. Sending money to a Bitcoin address or other account number without any other identification attached is a much easier mistake to make.
The certainty you associate with face-to-face delivery only exists if the two people know each other by sight, and is subject to exceptions--identical twins, for example, or simply people who look very much alike. If you sent someone out with money to be delivered, and gave them the wrong address or showed them a picture of the wrong person to deliver to, the situation would be very much the same.
If you knew who it was that you accidentally send the money to, would you not request the money back?
Certainly I would request it back. However, that would be an appeal to their good nature, not the assertion of a right.
Would it not be their moral responsibility to send it back?
Moral responsibility is a personal issue, and for them to decide. Ethically, I would say that there is no justification for
taking any of their property in compensation; that would be an escalation of force. If they ever erroneously send their property to you, however, you can feel free to hold on to it. Turnabout is fair play, after all.
Bitcoins are a bit of a special case, not being property to begin with. Keeping their actual nature in mind, they have no physical location and are not in anyone's possession; neither are they some sort of contractual claim, like a bank account balance. A balance in a Bitcoin account is nothing more or less than an informal agreement among most Bitcoin users that the holder of a certain private key has a certain number of bitcoins--an entirely virtual unit of account. Claiming that the bitcoins still belong to you after broadcasting a valid, signed transaction transferring them to another user's private key runs contrary to the nature of the system, and undermines its design.
Or, similarly, if the bank accidentally deposited $100,000 in your bank account, would they take it back? (Most certainly, they would, once the mistake was realized, as it was their money, not yours).
There are some differences here. One is that you're talking about whether the bank
would take the money back, not whether they
should, or whether they have the right to do so. Certainly they have the power to reverse the change, as the manager of your account. Bitcoin transfers are somewhat more permanent by design, and the block chain and private keys are supposed to be an authoritative record of ownership.
The second difference is that your relationship with the bank is governed by a contract, and such contracts typically include a clause nullifying transfers or changes resulting from human error. In other words, when you set up the account you agreed to return the money if they could show that it was given to you in error. A similar clause could be inserted into the standard account agreement for Bitcoin merchants, but it would naturally only apply to their customers, not anyone who happens to receive bitcoins from them. There are no central organizations governing Bitcoin accounts to enforce such terms among all Bitcoin users.
I think we're beyond laws here - I don't think anyone is arguing that any laws would apply here. It's more of an argument of ethics and morality.
Agreed.