If you are a big service provider, and give away lots of cheap (or free) services, you are bound to get taken advantage of. The blogging world has seen its fair share of blame, much at the hands of services like Blogger.
Frankly, search engine spamming via blogs is doing little to harm anyone except advertisers (and maybe a few researchers), so you could consider it more of a nuisance than anything else. But if you are unwittingly hosting thousands and thousands of phishing sites, you have a much bigger problem on your hands. And it seems that might just be happening over at Yahoo!.