Why not use USGS data?

I may be missing something here but why not USGS map data for even more detail?
I thought it was free to use, though maybe there are conditions of its use I am unaware of.

http://topomaps.usgs.gov/drg/

It is already done .Pls see the link for more details http://linux.com/feature/125344

Hmmm well I don’t see it when I look around the map in CA.
Link to a view with it please.?

“Free” if you’re prepared to pay for the servers required to host lots of reprojected background images, yes. I’m talking to a helpful guy about it at the moment - more soon hopefully.