This is presumably to prevent an issue in IE6, IE7 at the time that Magento CE 1.2 was released (ca. 2008) that prevented SSL downloads if
Cache-Control: no-cache HTTP/1.0 HTTP header was set. According to the Microsoft KB this may be an issue that presents all the way up to IE8, thus why it still exists in the codebase.
The (now defunct) Harapartners Blog points this out with a workaround - though I don't personally recommend it:
If you have SSL enabled for your site, downloads may fail for all
versions of IE, as IE contains a bug that prevents downloading over
secure connections if the no-cache header is set. This can be easily
fixed in an htaccess file by removing the no-cache and no-store
headers, or by forcing download links to be non-secure.