Because “do what your morals tell you to do” is self-referential, as your morals are what you should do. “Do what your morals tell you to do” unpacks to “do what you should do”, so if someone asks you what you should do, you can only respond “What I should do”. “Do what the RNG tells you to do” is not self-referential.
Because “do what your morals tell you to do” is self-referential, as your morals are what you should do. “Do what your morals tell you to do” unpacks to “do what you should do”, so if someone asks you what you should do, you can only respond “What I should do”. “Do what the RNG tells you to do” is not self-referential.