According to the Mother Jones article, the sand that's being raked up is sent to a facility where the oil is separated from the sand. The sand is sent to a hazardous waste disposal facility, and the oil is sent for processing.
In other words, they still plan on selling the oil recovered from the beaches. If that's the case, wouldn't it make sense to do the booms properly, so that all the oil wound up in a few small spots, and relatively uncontaminated by sand? It should be in BP's best financial interests to contain the oil before it hits the beach. So why don't they? What else is going on here?