They don't need to. People who park underground, walk or use public transit are close enough to people who park above ground that they get enough data without that. In most cases just one WalMart is enough to infer everything you need to know about them all - if you select the right one for your question. (you wouldn't select just one because then you need to ask if there is something specific about that one, but a statistical sample is enough).
Unless your thesis is people who park underground shop differently. That is a valid idea and you cannot answer it via this study. However you can study it by other means if you believe this is important.
Unless your thesis is people who park underground shop differently. That is a valid idea and you cannot answer it via this study. However you can study it by other means if you believe this is important.