It's a standard, not a rulebook. People can choose for themselves. This is a guideline.
For example, you brought up MUI. A lot of people hate this, but it is powerful, easy to code for and works on all systems (MOS/OS3.x/OS4). ReAction for instance is too weak to be a standard gui, and the improved OS4 version will only run on OS4. Not much use as a standard. I suppose if one was really anal they'd say 'use gadtools' - but you can see why that wasn't used. ;-)
