It could be that is coded in a certain way that only permits certain possibilities, namely windows decorations being rendered in a certain way buy certain rules.
Who controls the "renderer" controls the look, unless with CSD you mean allow any kind of renderer... and then complain why certain apps apply their own and its a "looking" mess -> crazy.
so its not like "gun point" and you have to abide, but simply coded to use what is included and no other option.
Don't like it ?... you can always fork KDE or Gnome (etc)... or pertinent parts and do it your way, only don't expect others to maintain it.