When Fastmail started in 1999, https (secure SSL connections over http) was still a relatively young protocol, originally released in 1994. It was considered fairly computationally expensive, and though technically supported by most popular browsers, there were numerous performance and compatibility problems (eg. early browsers never cached any https resources). So although we always offered https as an option via the "Secure Login" button, the default button was just a regular "Login" over unencrypted http.
Of course since then there's been massive changes in browsers, computing power and average level of security required on the Internet (eg. Firesheep). Because of this, https connections are now recommended for all logins, and we've defaulted to "Secure Login" being the primary login for some time.
As a further step, today we're making "Secure Login" the only button. There's really no reason these days not be using a secure login and having all data encrypted between your computer and our server.
For the very, very few cases where you might not want a secure login (eg. we've heard of some people in certain countries or on certain company networks having problems with https connections), you can click the +More link and there's a link there that will switch you to a non-secure login screen. We highly discourage using this, however it's an option of last resort if you need it.