Is there a name for this bad argument?
There's a particular annoying pattern I notice in debate, in which one person criticises another's choice of argument on the basis of a sort of misapplication of pragmatics.
Here's a concrete (if slightly melodramatic) example. Imagine we're drinking together, and you demand, suspiciously, ‘Wait, how do I know you haven't poisoned this bottle of wine?’. To which I respond, ‘I'm drinking from it too, so that would be a really bad idea!’ Now if you were to think, or say, ‘Oh, so you would have poisoned it if we hadn't been drinking from the same bottle?’, you'd be committing this fallacy.
Because in fact, of course, the main reason I haven't tried to poison you is some combination of because I don't want to and because I'm too moral to do such a wrong thing, and both of those reasons would still apply regardless of any detail of who was drinking what.
But you can't check those statements, because they're about stuff entirely inside my own head. So if I'd tried to use either one as a defence, then you'd be no more convinced of my non-murderousness than you are now, because if you can believe I might try to poison you in the first place, then you'd have no trouble also believing that I'd lie about my motives in the course of the attempt. Whereas you can easily verify for yourself that I am indeed drinking from the same bottle, and perhaps you might find it harder to believe I was self-sacrificingly murderous than merely self-interestedly murderous.
(Since this is a silly hypothetical example, let's assume we can disregard all the even more improbable edge cases beloved of fiction, like the poison being in the ice cubes, or smeared on your particular glass in advance, or that I took the antidote beforehand, or have spent ten years building up resistance to iocane powder, etc…)
Anyway. That's why I chose that particular reason as the one to mention: not because it was my core reason or my only reason, but because it was one you'd be more likely to believe, because you could check it yourself.
I think the general pattern of which this is an example is: it's a fallacy to assume that someone who has mentioned one good or bad property of a thing (or reason to do it or not do it, or whatever) must have chosen that particular property because it's the most important or the only one, rather than choosing the property most appropriate to what particular goal the utterance as a whole is trying to achieve.
In this silly example, my goal is to try to convince you that you're safe; so I have to pick a reason that will actually manage to do that job, rather than one that is more important to me but likely to be less effective. In other kinds of debate, one might similarly choose the argument that appeals most to the particular audience one is trying to convince, not the one that is most fundamental in the arguer's own mind. Or you might avoid particular arguments because you know they'll cause some enormous derailing sidetrack. Any number of reasons, really.
So. Does this fallacy have a well-known name?