Plenty of looters go after homes as well. Seriously, the people who lost their homes are usually not the people looting. Those people are too busy cleaning up the mess and trying to salvage what's left/run off looters to actually go looting the Wal-Mart.
I will agree that it is not the best use of the police department's time to protect giant corporations from looters, but people who decide to go rob somebody right after they get hit by a natural disaster, even if that person owns a Wal-Mart, are the lowest form of scum on the earth.