I'm a little unclear on how the Xamarin license works too. From what I understand, you don't actually need a license from Xamarin to use Mono. Somewhere in there is why you don't need a Xamarin license to use Unity 3D, which is also based on Mono.
I think the Xamarin license applies to just a few things - the IDE - Xamarin Studio, and some of their mobile libraries (though I think they recently made those available under and open source licensing). I once read somewhere that you needed a license from Xamarin to use Mono in a non LGPL compatible way, like building statically linked binaries for iOS - but I'm not sure how accurate that is.
The Starter Licenses from Xamarin are kind of like a bad joke - they should really get rid of it, because it comes off as kind of a scam.
Anyway, I'd love a detailed explanation if anyone has it. :-)
Kevin N.