Hi,
The reason for the error is because news.google.com is accessed via HTTPS. In any captive portal implementation, the networking device have to capture the HTTPS traffic and redirect it to a splash page. As part of this process, it has no choice but to hijack the HTTPS connection. Because HTTPS is a secure protocol specifically designed to detect such hijackings, one will see warnings like this in the browser. It is unfortunately not avoidable, one of the few imperfections in all captive portal implementations :)
Thanks,
Yan