I know that if you try to send from Blockchain.info without a miner's fee you get a warning that if there are zero confirmations after 24 hours it will no longer be sent. Danny, try for yourself if you don't believe me.
I know how blockchain.info works, I also know how the bitcoin protocol works. You appear to only be aware of one of those two things.
As for acceptable risk, tell the average Wal-Mart shopper that not sending a miner's fee may lead to them getting to keep their money and see how many choose to pay the fee.
Tell the average Wal-mart shopper that they can report their credit card stolen, or pass a bad check, or manufacture some fake U.S. currency, or simply walk out with the merchandise without first paying for it and get to keep their money. What's the difference? Fraud is fraud, doesn't matter if it's with credit cards, counterfeit cash, paypal, checks, or bitcoin.