Inaction is an indifferent state of being. It's neither choosing action, nor choosing not to engage in an action. It's just... being.
However, choosing not to engage in aggression, is a moral choice. It is not a neutral thing to say, "I choose not to engage in violence against you, because it's wrong to do so.".
Since violence/aggression is one of those things that you have to either choose to do, or choose not to do, I don't really see how it could be categorized as neutral to refrain.
The basic positive statement of theory would be something like: "It is right to engage in X behavior.", or, "Anyone who engages in X Behavior, is 'good'", where in this case, "X Behavior", is "refraining from commiting unprovoked acts of violence against other individuals".
We know that a moral rule is only valid if it can be successfully applied to all people at all times and in all places (universality), and if it is logically internally consistent. Since we assume the default "correct" state is goodness for all men, then any action that would be impossible to achieve universally, could not be said to be "required" for men to be good.
This rule is asserting a requirement for "goodness" that is possible for all men, everywhere, at all times. This rule succeeds the validity test then, since refraining from an act of violence is not extraordinarily difficult (even babies can do it), and does not endager the life of the individual in question. Therefore, the NAP is UPB compliant (moral), and anyone who engages in refraining from acts of violence is therefore, "good".
Does that make sense?