I work for contractors exclusively.
the contractors check your info and the customer goes over it again. they review the lists see who you are bringing onto their property, check your licensing, your I-9's, date of your last u/a, are all your saftey certs up to dat, welding papers curent.
you are liable for what happens on your property and if your worth a damn as a bussiness men you check, you are responsable to provide a safe work place, you are responsable to make sure no illegal bussiness practices are happening on your property, you're taking the liability you better know whats going on.
I'm deffinately not a fan of walmart but you won't convince me that they are sloppy bussiness men. they know how to take care of their bussiness, and if this was going on at several stores they either knew, or should have known.