I think you are being disingenuous if you think your bank don't do similar things. Ever had to bank a cheque? When you deposit it into your account they immediately debit the account of the person that wrote the cheque, does it appear in your account for spending immediately? No you wait 3-5 working days, what happens in the interim? The bank hold it and accumulate interest on it and then you are allowed access to it.
And if the person who had their account debited would drop to a negative balance at the end of that 3-5 days the check will bounce and they will be credited back the amount of the check.
Edit: You of course do not get the funds in the case they are credited back to the original person at the end of the 3-5 days, you do probably get a fee for trying to deposit a bad check though!
