A project by the U.S. General Services Administration that outlines how Federal agencies should implement basic web security, going into great detail as to why government websites should always use HTTPS over HTTP:
HTTP has become central to today’s way of life. HTTP is currently the primary protocol for applications used on computers, tablets, smartphones, and many other devices. As our dependency on the internet has grown, the risk to users’ privacy and safety has grown along with it. Every unencrypted HTTP request reveals information about a user’s behavior, and the interception and tracking of unencrypted browsing has become commonplace. Today, there is no such thing as non-sensitive web traffic, and public services should not depend on the benevolence of network operators. When properly configured, HTTPS can provide a fast, secure connection that offers the level of privacy and reliability that users should expect from government web services.