Normal Googlebot IP addresses resolve to names like crawl-blah.googlebot.com. These ones don't. You could use whois to detect them, but it's a lot slower than DNS. I guess that Google expects the SEO guys not to be smart enough to use something like pwhois to quickly check that the IP's on a Google AS (likely).

There are some google ip addresses. Who knows how many google but registered in another company's name IP addresses they're doing this sort of thing from

To be fair to Google, they do get inundated with spammers, so a certain level of distrust is understandable. The problem is that like Pierre mentioned this is very easily gotten around, which makes it fairly ineffective, and to anyone who actually uses traffic logs for any kind of stats tracking, any fake referrers can be quite annoying.