function bbfontstyle has the lines :

    } else if (textarea.selectionEnd && (textarea.selectionEnd - textarea.selectionStart > 0)) {

        mozWrap(textarea, bbopen, bbclose);

        textarea.focus();

        theSelection = '';

        return;

    }

When the selection is of non-zero length, the test is true and this clause is executed. mozWrap is undefined, so nothing is done. The selected text is not wrapped with the style. The affected BBCode does not work.