Danny, if there is a way for the fee to be added, via the technology, so that when I send the BTC I cannot choose to not add the fee, then I will be happy to stand corrected. I know that with blockchain.info I can choose not to add it. It is entirely up to me, and if I say no, then it doesn't happen. The person receiving the BTC has no say in it. Maybe I am wrong, but I don't think that I am. They can choose to not give me what I paid for until confirmation, but that is all.
Also, with Blockchain.info unconfirmed transactions get cancelled after 24 hours.
Thus, if I were to use that wallet to buy something at a store I could not pay the fee and I *might* get a free lunch.
Zero confirmations is definitely not instant under these circumstances. Zero confirmations simply means that the transaction may go through, and with no miner's fee paid there is a good chance it won't.
Nobody is denying that 0 confirmations is susceptible to fraud.
0 confirmations can be instant if the buyer sends the transaction through the sellers network, so that the seller can see the transaction.
Also, the seller can simply accept that the buyer has paid without proof. There are examples where trust is a reasonable risk. All of my direct bitcoin sales are 0 confirmation, and some people don't even bother to wait to see the transaction on the network.