Most if not all Walmart stores have banned the bringing in of backpacks on your in the store and some of them have signs posted at the entrance of the stores.
I believe the reason on the ban on taking in backpacks in the store is due to shoplifting where people put things in their backpacks and walk out of the store without paying for the items.
The most that will happen is if you do try to take a backpack in then the management may stop you and tell you to take the backpack outside.