The profit margin for Wal-Mart is not that great but the increased costs could be passed on to the consumer.
Yup, if Wal*Mart unionized, that would likely spell the end of Wal*Mart. The only way they'd stay in business is if all other large retail was unionized as well. And if they were, goods would cost substantially more. There's a reason why shipping a package UPS is so expensive.
Just like these idiots at McDonald's demanding $15/hr. To flip burgers. What do you think would happen to McDonald's? How much would a #1 cost? More than most people would be willing to pay. It's already too expensive. $8 for soda (worthless), fries (nearly worthless), and a cheap burger with the only ingredient worth much, the low quality beef. Buns (dirt cheap), condiments (pennies), veggies (pennies). The majority of the cost is the labor and the land/building/operating cost.
Now nearly double the labor cost and these restaurants have problems.