TerraServer explicitly allows access to their USGS map database from programs like ExpertGPS. They even have a webpage with step-by-step instructions on how to do it.

ExpertGPS could just as easily grab its maps from sites like TopoZone and deprive them of ad revenue. Other programs have actually done that, and caused the nice guys at TopoZone a lot of hassle and lost revenue. The guys at Geocaching.com spend lots of time dealing with database scrapers who mine the site continually, chewing up bandwidth.

The moral of the story - play nicely. If a website like TerraServer is generous enough to offer you a way to scrape their data, say thank you. If a website asks that you refrain from using automated scripts, either work out a licensing agreement with them, or start your own website and learn how it feels to be on the other end of the scraper.