I don't see a reason to have a unique menu for right clicking a function shader. I mean at face value it seems good to just have the function menu right there because you are working with functions, but sometimes you to add other shaders, such as a smooth shader, or a surface layer for basic masking, etc. These options simply aren't available.
And to counter the unique menu, the very first option closest to your mouse when clicking any other red shader is the Function menu, so it's not like it's hidden, far away, or in any way really inconvenient to access.
It would be cool to just use the global menu system on functions in later releases, omitting the few shaders that don't apply like shadow catcher, etc, or just leave them and have TG do nothing since it shouldn't if I recall.