Google has known for a while that millions of websites registered freely (or very cheaply in bulk) through a Korean company are plagued as malware-hosting sites. The most notorious subdomain full of these types of sites was .co.cc.
Kicking things into spring cleaning mode after rolling out several new products and freshening up existing ones, they knew they'd have a hell of a task on their hands isolating the bad sites from the good, especially when you're talking 11 million of them!
Easiest solution? Block the entire .co.cc subdomain of course, which is exactly what Google has just decided to do. As to how effective this drastic manoeuvre is long term, I can't imagine all that significant in line of the seemingly forever resident global malware war.