The U.S.G.N. itself never actively pings any servers. It uses a heartbeat system. Servers ping the U.S.G.N. every 45 seconds to tell it that they are still alive. If a server doesn't ping for 3 minutes it is removed from the list.
This approach is more efficient than what you suggested.
The usgn.php website is just a read only representation of the server list. It has no effect on the actual server list in any way and is not a part of the U.S.G.N. master server at all. Info requests for details are only executed on demand (when a user clicks a server) and are cached for a few seconds. There is no optimization required for that either. Also pinging all the servers regularly just for that page would cause much more traffic and would be less efficient.
That page is basically a little extra to give people the possibility to check the status of a specific server online. It's not meant to be a replacement for the much more powerful in-game server list.
(the technical problem with the web server list is that all the pinging has to go through the web server which is not cool. Unfortunately it's still not possible to open plain UDP sockets in web browsers via JavaScript. Otherwise I could ping the servers directly via the client connection just like in the game)
@ MikuAuahDark: I can't simply enforce that because of CS2D. I'll check if I can enable it for everything but the CS2D client. Thanks. Actually... I guess the CS2D client just ignores that header anyway so it shouldn't be a problem.