Mono is a completely different implementation from the CLR. I wouldn't be so quick to discount the CLR's GC.
Here are the conditions I could find (you still have to comply with those conditions, but it's doable): http://msdn.microsoft.com/en-us/library/ms973265.aspx
And they can stop you through the EULA .NET gets distributed under, because no other license gives you the right to use .NET, so you have to do it under the terms of that license.