So what is "becomes aware of a link"? How is that different than the copyright holder sending a notice? Because without the copyright holder (and even courts) it's not very clear.
I am not a lawyer. I believe Google is exempt because it indexes all links it can find. It needs this to operate a general purpose search engine. As soon as Google became a specialized torrent search engine they could be similarly liable. Also Google has to comply with legit DMCA requests or they still risk being liable.