This is not true.  The merchant does not know that the payment is safe, instantly or otherwise.

I am familiar with a website that accepts credit cards and provides music downloads.  When the credit card processor gives preliminary approval the web site enables the download. Rarely, a fraudulent or dead-beat "customer" can complete his download before the credit card processor revokes the preliminary approval.  The web site never gets the funds, but the customer has the electronic goods. With this credit card processor it is possible to reduce the probability of loss by waiting longer, but this takes several hours (and there is still a charge back risk).  The website owner made a business decision to deliver the goods immediately, for customer convenience.   So far, this has proven to be a good decision.

How is this different from accepting bitcoins  with zero confirmations?