You make a fair point and I normally share your view about compilers and tools. I think the Xamarin situation is complex, though, because most of the product is already open source. The C# compiler and x86 runtime, the standard libs (System.*), and the IDE (monodevelop aka Xamarin Studio) are all open source; and the platform bindings are a thin, almost 1:1 layer covering the iOS and android APIs. What you're really paying for is the AOT mechanism, packaging, and ARM runtime -- it's about all that's left they can sell. It's also worth noting that a ton of their code goes back into mono and associated projects.
I still think you have a good point, and I agree, but as for myself, I decided to make an exception in this case. I don't think it's quite as bad as PowerBuilder or Clarion at least.