Why does Wal Mart have people gather in the weakest part of the store during a tornado warning?

The general practice for a Code Black (severe weather/tornado warning) is to gather all the associates and customers in the middle of the sales floor--structurally the weakest part of the store, as the recent Joplin tornado proved. The safest part of a Wal Mart store during a tornado is the rear bathrooms. So why is it not company policy to gather everyone there?