IP addresses aren't reliable, especially when considering spammers will know IP address geolocation will be used. It may be safe to assume a vast majority of US IP addresses will actually be proxies.
When accusing domain owners of spam, don't forget that anyone can spam thousands of blog comments for $5 using any domain (think negative seo). For example, I really doubt that Steve Pavlina (#3 in the spam domains list) would spam his own domain. Most probably some competitor wants to take his domain out of the google index.
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[ 3.5 ms ] story [ 18.7 ms ] thread#1 USA (30%)
#2 China (20%)
#3 Russia (6%)
#4 Brazil (4%)