1) C# is a standardized, open spec language. Mono is an implementation of that open spec. The development of that specification is driven by MS's .Net platform.
2) Here's a crucial bit of patent/IP law: by releasing the specification of the language to the "public", MS is barred from suing over it in a court of law. So again, there's no need for it say that it won't sue, since it's already prevented from doing so.
And even if by publishing the spec they would have given up their right to sue implementors (which they did not) they would be free to sue anyone that used any technique on which they own valid patents on any jurisdiction insane enough to allow such patents.
And while I doubt their willingness to test such patents in court, I also acknowledge the potential for long and painful lawsuits is more than enough an incentive use other technologies that are every bit as nice to the developer as .NET while negating completely any litigation risk.