Here's a howler from an expert quoted by the WaPo on GeorgeWBush.com being inaccessible overseas:
"I've never heard of a site wholesale blocking access from the rest of the world," said Johannes Ullrich, chief technology officer for the SANS Internet Storm Center, which monitors hacker trends. "I guess they decided it just wasn't worth the trouble to leave it open to foreign visitors."
I guess it doesn't count if it provides a counterexample, then. Both sites have good reason to block access; Showtime for legal reasons, as they only have the rights to show clips of their programs to American audiences, and the Bush campaign site as a means of proactive defence against attack. The cost, that expat absentee voters won't be able to see the site, was apparently calculated to be less damaging than domestic voters being unable to access it due to foreign DDOS attacks. It's not rocket science, just a simple cost-benefit calculation.