Custom Header Image in WP 3.4.1

When registering custom header image with 'flex-height' => false I expected that custom image uploaded trough theme customizer would be cropped if it exceededs ‘height’ parameter of add_theme_support for custom header. But crop dialogue did not appear. Is it intented behavior?

This is my custom image registration code

$args = array(
'default-image'  => 'my_url_here',
'random-default' => false,
'width'   => 980,
'height'  => 300,
'flex-height' => false,
'flex-width'  => true,
'uploads'     => true
);
add_theme_support('custom-header', $args);

With this code I’d like to prevent usage of very tall images in header when they are uploaded by user through theme customiser. Did I get it wrong and 'flex-height' => false does not forse images to be cropped? Could anybody please explain the logic behind these settings?

Thanks a lot in advance for any advice,
Anna