Interesting but useless fact

According to the fount of all knowledge  -  Wikipedia ;-)    - the only three countries not to have adopted the System Internationale units are Burma/Myanmar, Liberia and the United States of America. 

I can't help thinking that there is something deeply significant about those three countries falling into the same group, but I can't quite put my finger on it.

One of my big thick first year university physics text books has in large red writing on the back cover - "Not for sale in the United States".  I assume this is because of the SI unit issue.  I also assume it will be hard to find this book in a bookshop in Monrovia, though I haven't tried looking for it there.