I think Netscape was a clean rewrite of the source code of NCSA Mosaic, but with the benefit of past experience and being able to look at the code. Not illegal, but still somewhat dodgy, and understandable that people who worked on the NCSA version might get mad. I remember hearing that Andreesen was none too popular at UIUC after founding Netscape, and the lawsuit over the trademark; there's a source for that here:

https://www.jwz.org/doc/about-jwz.html