That about 1 Starbuck for every 600km², hardly a replacement for public bathrooms.

of course the vast majority of the US is open land, still, far from any Starbucks or humans, so the distribution of Starbucks places them close to humans for some huge percentile of humans at any given point in time in the US, by design of course.

Sure. How many public restrooms do you think USA should have?

More than Starbucks alone can or should be forced to provide. Where that number lies I'm not entirely sure, but I imagine interstate commerce and population densities would be a pretty good starting point.

1% of the population, so around 3 million, would be a good starting point.