What do you think the cost to the taxpayer would really be if the USGS let people download high-quality PDF (or some other image file) version of their 1:24,000 quadrangle maps instead of just offering little JPEG thumbnail images and selling print copies for $6.

You're talking some number of lost sales to people who would otherwise have bought the print copy. But not a lot of lost sales. The kinds of things people are most likely to do with these maps -- take them hiking or sailing or kayaking; hang them on a wall -- aren't well-suited to electronic media. And there'd be some cost associated with the bandwidth. The service, meanwhile, would be potentially quite useful to at least a few people, and would open unknown doors to the enterprising.