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Hello Jordan,

I'm not sure your diagnosis is accurate. Joomla! sends cache busting HTTP headers in the back-end login page. Here's the response from accessing the administrator/index.php URL on my dev site as recorded by Charles Proxy (minus the HTML part of course):

HTTP/1.1 200 OK
Date: Wed, 11 Nov 2015 14:39:39 GMT
Server: Apache
X-Powered-By: PHP/5.6.10
X-Frame-Options: SAMEORIGIN
Expires: Mon, 1 Jan 2001 00:00:00 GMT
Last-Modified: Wed, 11 Nov 2015 14:39:39 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Keep-Alive: timeout=5, max=96
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8

The Expires, Cache-Control and Pragma headers tell browsers and caching proxies to NOT cache the page.

The other back-end pages also send similar headers:

HTTP/1.1 200 OK
Date: Wed, 11 Nov 2015 14:40:22 GMT
Server: Apache
X-Powered-By: PHP/5.6.10
Expires: Mon, 1 Jan 2001 00:00:00 GMT
Last-Modified: Wed, 11 Nov 2015 14:40:23 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Keep-Alive: timeout=5, max=99
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8

Please note that these headers are sent by Joomla!, not the .htaccess file.

I checked the front-end pages with and without caching enabled in Global Configuration. They still return headers to bust caching, therefore overriding the default cache handling in .htaccess. See:

HTTP/1.1 200 OK
Date: Thu, 12 Nov 2015 07:46:51 GMT
Server: Apache
X-Powered-By: PHP/5.6.10
Expires: Mon, 1 Jan 2001 00:00:00 GMT
Last-Modified: Thu, 12 Nov 2015 07:46:51 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8

When you enable the "System - Page Cache" plugin AND its "Use browser caching" option then you do indeed get no Expires and Last-Modified headers which does tell the browser that it should cache this page. However, even if you disable .htaccess Maker's expiration time feature the page will still be cached by an amount of time specified by the browser. That is to say, the default expiration time does not seem to have any effect whatsoever to the Expires headers.

You may want to try adding

ExpiresByType text/html "now plus 1 second"

in the "Custom .htaccess rules at the bottom of the file" area and regenerate your .htaccess. This will effectively instruct the browsers to NOT cache your HTML pages which completely counters the purpose of setting the global cache setting but is equivalent to removing the default caching line.