One problem is that copyright is not well-defined. They just have to convince a court that their use is "transformational" and not a "derivative work" and they're home free. Then it doesn't matter what the license says, because Microsoft isn't licensing your work.
Also, some people explicitly avoid reading codebases to avoid duplicating by mistake some copyrighted code.
Not sure if this is true but I heard that it used to be forbidden to Microsoft developers to read open source code.