1. Stores that offer free parking have more customers 2. Stores that have more customers can offer better prices to all their customers (including poor people) due to greater sales volumes
A large majority of US metro areas mandate huge amounts of free parking that are enough to have space in Black Friday, whether they need it or not. For instance, there's a strip mall near me divided in two halves. One half has a neighborhood supermarket, three tiny stores (hair salon, gamestop, froyo), and two restaurants. That half has 400 parking spots. Typical number of spots in use? well under 100. It's not even half full when every line in the supermarket is working right before thanksgiving. But those 400 spots are mandated by area, just because the municipality finds all this parking space a great way to deter the least successful merchants to move in: Making sure all space is premium space is a way to make sure poorer people not only don't move to the municipality, but not even come here to shop. A bit like how municipalities also don't want public transport nearby.
Which places around here lack public parking? Urban areas where poor people live, as it's a way to tax non-residents.
Therefore, if this was really about what is good for stores, we'd see a lot less regulation, as opposed to what is good for the municipality itself.