HOUSTON—At midnight on the first of the month, a scene unfolds at many Wal-Mart Stores Inc. sites that underscores the deep financial strains that many low-income American consumers still face. Parking lots come to life after 11 p.m. as customers start to stream into the stores, cramming their shopping carts full of milk, infant formula and other necessities. Then at midnight, when the government replenishes their electronic-benefit accounts with their monthly allotments of food stamps, nutritional grants for mothers with babies or other aid for needy families, they head for the registers. "We're not starving or anything, but we come...