Part of that reason is because Buschwick handles it a little different. Pretty neat, my opinion, how it handles the header/featured image.
What it does is run a quick check to see if it is a single post and if it is it will use the post-thumbnail if there is one. If there isn't one it won't use it as the background image.
One way I can think of would be creating a child theme, and passing it some information. It all really depends on how comfortable you are with code. :)
I tried this:
add_filter( 'bushwick_custom_header_args', 'child_args' );
$args = array(
'wp_head_callback' => 'child_callback'
// What will be used instead
The downside is, of course, that we don't really have access to your plugins code so I can't really say what to use inside the callback function. :)
Hope that helps you out at least some. If not, don't hesitate to ask questions. :)