It might be useful to distinguish between a "moral theory" which can be used to compare the morality of different actions and a "moral standard" which is a boolean rule use to determine what is morally 'permissible' and what is morally 'impermissible'.

I think part of the point your post makes is that people really want a moral standard, not a moral theory. I think that makes sense; with a moral system, you have a course of action guaranteed to be "good", whereas a moral theory makes no such guarantee.

Furthermore, I suspect that the commonly accepted societal standard is "you should be as moral as possible", which means that a moral theory is translated into a moral standard by treating the most moral option as "permissible" and everything else as "impermissible". This is exactly what occurs in the text quoted by OP; it takes the utilitarian moral system and projects it on a standard according to which only the most moral option is permissible, making it obligatory.