>> they will of course let the company know that "We're boycotting you because of this principled reason!"
> But this claim will clearly be false, and so they will not be credible.
This is where you're wronger than a $3,50 bill, so glaringly not-even-wrong that it baffles the mind that you can't see it yourself.
To begin with the lesser reason: "clearly". Clear how, why, and to whom? For the umpteenth time: How would the boycottee know anything about this? Maybe you have participated in more boycotts than I (not a very high bar to cross), so please tell me: Is one usually required, before being allowed to tell Nike "I'm not buying any more of your shoes as long as you keep using third-world child labour!", to declare which other companies one is boycotting and on which grounds -- and above all, which ones one isn't boycotting? How does it work, in practice; is there a form to fill out? Does each company that gets boycotted make up their own form, or is there some central registry? Or is it just hired goons that nab any picketers outside the headquarters and give them the third degree? This is anything but "clearly" false.
Mainly, of course, because it isn't fucking "false" at all. Oh sure, when I was a kid I was also very "principled". But then I grew out of my teens -- or into them? -- and realised that that isn't how the world works. I'm sorry if this comes as news to you, but there is such a thing as differences of degree, and they matter. As somebody[1] is supposed to have said: "Quantity has a quality all of its own."
That is, things can be more bad or less bad, and since nobody has the time or energy to care -- or at least, to do something about -- all of them, people pick and choose which ones they care the most about, and do something only about those. This is perfectly normal, valid, logical and correct. Everyone does it: You too. Say you don't, and we'll know you're lying.
Kids, and perhaps people on the infamous "spectrum", can't distinguish between what's important and what's much less so. This leads to the fallacy of "If you don't do something about everything, you're not allowed to be against anything!" If you're a fully-functioning adult seeing it spelled out this starkly you'll realise how fucking wrong it is. If you're very young, you will when you grow up. If you tend towards autism, this was your lesson for this week on how we neurotypicals see the world[2].
I mean, we're all against Bad Things, right? We think people who do what's obviously wrong should be punished, or at least severely reprimanded. Have you ever uttered your displeasure with some, say, rapist, drunken hit-and-run driver, drug dealer, or genocidal war criminal? Even if you didn't even write a letter to your political representative; just being one more voice contributing to the general opinion in the break room at work, you might have put the last grain on the scale that made someone else write in, right?
But that's about Really Bad People. When did you last go out waving placards -- or even just go on a bit of a tirade over a cup of coffee -- about the evils of, say, occasional littering, jaywalking, or riding a bike without a helmet?
Naah, didn't think so.
Q. E. fucking D.
> Call it capricious rather than arbitrary if you think that's important;
The only one who does seems to be you.
> That's not a request to explain my motivation, it is a request to give (IMO intense) precision.
On the contrary, it is clearly (Hah!) a request to answer a binary, black-or-white, yes-or-no question: Do boycotts, irrespective of your imputed "arbitrariness" of their motivation, still concern the boycottee? The commenter was probably going to continue in the vein of "Even if it is a bit less, who gives a shit? They're still concerned, so they'll have to do something about it." But hey, congratulations, your Sheldonning seems to successfully have deflected that. Are you satisfied with this? Even proud, perhaps?
Don't be.
___
[1]: Often attributed to Stalin, IIRC.
[2]: I gather a rather typical strategy in order to "fit in" is to learn to either understand the reasoning or, if one simply can't, to just bloody fake it. Free tip, worth every penny you're paying for it.