> Why is it ok for people to shop at Walmart and not at their local small business that sells the same goods and is better equipped to enforce social distancing, with customers explicitly opting in to visit the smaller store?

Small local businesses in the Bay Area that sell similar items to Walmart are open right now, from hardware stores to produce markets. I imagine apparel stores are shut, though many around me are doing online and appointment apparel shopping.

If Walmart is the only store open where you are, it's likely the only store that exists selling essentials where you are, and that is a different problem.