1) malware is often very aggressive and fast-spreading, and once it's on a user's computer it's hard to get off, therefore...
2) the system to detect it and stop access to the site has to be automated, not a human-in-the-loop system that might take hours or days to shut off access to a site which is infecting many users per minute, and...
3) the more clarity there is on how exactly that automated system works, the more certain we can be that malware will be able to evade it; it's much like how spam detection or search page rankings are opaque, because the incentives to game the system are very great
I'm not saying Google's system is perfect, but I am saying it's a very hard problem to solve in a way that doesn't give us an even worse time stopping malware spread than we already have. So while it is hard to feel sorry for a company as wealthy and powerful as Google, I think the issue is not as clear-cut as some comments on this thread seem to suggest.