(even though it strikes me as a mind projection issue of creating a person's ethical 'character' when all you need is the likelihood of them performing this act or that).

It's not a fallacy if the thing your projecting onto is an actual human with an actual human mind. Another way to see this is as using the priors on how humans tend to behave that evolution has provided you.

But on another level, I can see it is as a description of a sort of hard-coded irrationality that we have evolution to thank for. All things being equal, we prefer to associate with people who will never murder us, rather than people who will only murder us when it would be good to do so - because we personally calculate good with a term for our existence. People with an irrational, compelling commitment are more trustworthy than people compelled by rational or utilitarian concerns (Schelling's Strategy of Conflict), because we are aware that there exists situations where the best outcome overall is not the best outcome personally.

The definition of "rational" you're using in that paragraph has the problem that it will cause you to regret your rationality. If having an "irrational" commitment helps you be more trusted and thus achieve your goals, it's not irrational. See the articles about decision theory for more details on this.