Not a horde/IMP bug as such. Take a look at /home/httpd/html/horde/imp/config/prefs.php; you should see a section that looks like this:
// Should the original message be included?
$_prefs['reply_quote'] = array(
'value' => 1,
'locked' => false,
'shared' => false,
'type' => 'checkbox',
'desc' => _("Include original message in a reply?")
);