Actually that's not true, you receive rares even if you aren't prem, but the chances increase a lot if you are premium.
As far as I know the "Afterwind" rare unit isn't available for non-prem.
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Here is a link to all the rare units
http://atwardatabase.weebly.com/rare-units.html . I though it includes where they are dropped but apparently it doesn't.
If I remember right Child Soldiers are dropped in Africa and South America, Sky Infantries in Scandinavia,Foreign legion in France, Partisans in places such as Cuba and ex-Yugoslavian countries.
If you guys took the whole world while it was on Rare Units: High though, guess it's just bad luck