The issue is when they didn't just fork the codebase, they repeatedly flew the person who wrote the code out to Seattle on false pretenses, implying a job offer and additional money for their work, then picked the developer's brains until Microsoft learned all they needed from him, then just ignored all communication from him.
The issue is that it was dishonest and scummy behavior.
And you keep the copyright notice, otherwise you're violating the license.
If it's based on the same design (i.e. same file formats, mechanisms, etc.) then the issue is still not giving credit and pointing out the design it's based on. Doesn't cost them a thing and gives a lot of goodwill from people.
Like, if someone uses my code, I'm happy, if they copy the idea and present it as their own that's dishonest.
And lets not forget - they had similar open source project and community. They decided not to participate but create their own. This new project will overshadow existing and eventually kill community.
The issue is when they didn't just fork the codebase, they repeatedly flew the person who wrote the code out to Seattle on false pretenses, implying a job offer and additional money for their work, then picked the developer's brains until Microsoft learned all they needed from him, then just ignored all communication from him.
The issue is that it was dishonest and scummy behavior.