Businesses like Walmart or Target have large enough buildings you'd have to go inside. Same goes for malls probably. Not everyone who goes to the pokéstop is

Businesses like Walmart or Target have large enough buildings you'd have to go inside. Same goes for malls probably. Not everyone who goes to the pokéstop is going to spend money, but even if 1% of people who visit buy something, the pokéstop is still generating far more value than any other kind of online ad.