When you submit the ajax-modal form it sends the form using post method.
even when i tell him to use :method => "get" because
In the assets "lightbox_dialog.js" uses post for forms with class ajax_form. 

look at those lines in (lightbox_dialog.js):

 $('#ajax-modal').html(data.responseText);
            Blacklight.setup_modal('.modal-footer a', '#ajax-modal form.ajax_form', false);

...

 $(form_selector).submit(function(e) {
      var jqxhr = $.ajax({
        url: $(this).attr('action'),
        data: $(this).serialize(),
        type: 'POST',
        dataType: 'script'
     });