I am surprised. I always thought that increasing low salaries was good for the economy because those people generally put 100% of their take home pay back into the economy. On the other hand I put a large percentage of my pay into savings (including 100% of the differenc when I get a raise) which has a much smaller benefit to the local economy.
But in this case, if they buy books, they will buy from Amazon, which won't have to increase its prices when San Francisco raises minimum wage, as that will have 0 effect on their costs.