Xamarin has quite a few products based on Mono which they sell. A separate implementation of .NET and the BCL doesn't benefit Microsoft that much, but the other offerings Xamarin has might.

.NET is merely one of the implementations of the ISO/IEC 23271:2006 standard (http://www.iso.org/iso/catalogue_detail.htm?csnumber=42927)

Mono is also an implementation of that standard, a cross platform one, and open sourc-ish (they do some dual licensing stuff)