In retaliation against Amiga, Inc. (now known as Amino, Inc.), MUI was made available for free to any MorphOS user. A similar retaliation was made by the CyberGraphics developers when Picasso96 was chosen as the default API for graphics cards.
Where did you get that little gem from? CGX was part of MorphOS before Amino/Amiga Inc. even got founded and MUI was part of it since its earliest days, has nothing to do with any retalliation or any other conspiracy theory.
Also, MUI is not "free for MorphOS users", it is a core part of MorphOS, paying for MorphOS pays for MUI.