Spam in short supply in West Africa?

What is it with the Nigerians?
Nearly all of the infamous trolling spam I get in my mailbox purports, at least, to be from someone in Nigeria. (Though why you would pretend to be from Nigeria is beyond me, though there may be spam wizards out there who think Nigeria is the place to be from...)
Either they are very stupid (unlikely, though possible), or they think we are very stupid (probable, and may actually be true), but either way they are certainly painting the Nigerians as duplicious and untrustworthy people.

Rico says we should just permanently cut Nigeria out of the internet and solve the problem, but then we'd all just start getting spam from Sierra Leone...