And why don't they do the same with DNS queries?

Quoting from the recently released public DNS service:

"The temporary logs store the full IP address of the machine you're using... We _delete_ these temporary logs within 24 to 48 hours."

http://code.google.com/speed/public-dns/privacy.html

Is there anything in the Patriot Act that says explicitly that search engines should keep search logs but not DNS queries?