[elcomet]

But what matters is the output, no ?

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.

[sp332]

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.