"To my mind" isn't really a legally defensible position.
It's unlikely Kairosoft, or any company, will sue in the US without first notifying. Courts much prefer you solve things without taking up their time, and so most company lawyers know to first give notice.
Given that, it makes perfect sense NOT to ask for permission. If Kairosoft is unaware of it, or okay with it, for the time being at least, they can just ignore it. If that stance later changes, they will almost assuredly notify (especially given the international nature of it, since an international court case, against fans, for something they're not selling, is going to be a lot of trouble and expense and loss of good will, for no real gain) in which case the translation can be taken down.
So legally, this is the sounder strategy. But you may be talking about the ethics of it. We'd have to define what we mean by ethics before really diving into that, but consider - if they were to ask permission, even if everyone involved in creating the game, even if the CEO of the company, were all on board letting them translate it...the lawyers likely wouldn't be. Actually, the lawyers might WANT to say yes, too, but feel it their legal duty to say no - "If we give them the okay others might etc etc and it would weaken future cases in court and etc etc". And based on that the company would say no, even though -everyone involved- there wants to say yes. Not asking, then, gives the company plausible defense later should they need to assert their copyrights, without actually telling the translators 'no' now.
In short, not asking, regardless of how the company feels about it, risks only that the translators will later have to scrap their work. It does not risk the company anything (and in fact gives them the option to allow their work to reach a broader audience AND to assert their copyright any time they choose). As long as the translators are okay with that risk, it presents a win-win.
Now, what makes sense to do -in Japan-, I couldn't say.