I'm sure there were some lawyers involved in this decision, but if you limit your developer pool to people who know .Net and haven't ever felt the desire to take a look at how it works, then you might not have the best developers to choose from
Well, you surely would not get those who like to copy-paste their code.
Seriously, are you sure .NET source code is all rosy? I bet they had to clean it up quite a bit before releasing for everyone to see, if the previously leaked Windows 2000 code is any indication (I haven't seen it, just read this review).
From the initial announcement, a snippet of commented .NET source:
// restore culture
RestoreCultures(currentThread, prevCulture, prevUICulture);