This error message, I believe, was coopted from an older system Google used in 2005. At that time, a virus was making its rounds which caused the infected computer to run automated queries against Google looking for vulnerable servers. Google was able to spot the virus-originated queries and would show this error message.
It looks like they updated that old system to detect automated activity, but give visitors the benefit of the doubt by displaying a somewhat cryptic error message that insinuates the block is due to a virus or malware rather than an
SEO tool.