> for very high traffic websites HTTPS costs a lot of money. .gov sites are not very high traffic.

.gov sites can be very high traffic. Think weather, social security, health care, immigration, visas...

> - When you have such high traffic you also have a bunch of old winxp and similar clients. These dont work fast on HTTPS and dont work without sslv3 and what not.

SSLv3 only kills IE6, which is doable. And it's okay if very old clients work slowly with HTTPS -- those clients have far more problems than just slow HTTPS.