I think it's downstream of "you can't optimize for two different objectives".

If you only have functional requirements, then in effect you're doing some form of program synthesis, and RL can optimize that very hard.

If you have a mixture of functional and non-functional requirements, you are basically giving the model an incomplete specification, and it must in some way guess at the user's intent to fill in the blanks. This is also why adding to the prompt examples of the style of code you want (hats off to antirez for this particular tip ;)) is phenomenally powerful.