While it's true that the game is the dev intellectual property, depending on country law and international law, the fact that the translation is someone else work (nothing prevent you legally - at least in most country - to modify a game, but you lose all warranty to that game) might be defended in a law court - probably tricky but it might end up in a trial, even if not in a victory for the translator.
I'm not concerned here about the legal point of view - but about the impact on the players / translator community.
If dev start grabbing free fan made translation without consent of the translator, it's not a real positive signal- peoples could see it as a way to make more money, making the dev earn the ire of the westerner community - more pirating than buying to support, less translation fan made - or hidden behind paywall...
That's the "bad end" outlook on thing.
But considering that lately more and more H game get translated and sold on many platform (dl site eng, mangagamer, jast, nutaku, steam...), suddenly stealing fan made translation might turn the westerner public from happy to defiant.
Most fan translator are doing the translation because they like the game, not for money. While money is always welcome, I got the feeling that most translator wouldn't ask for money if the dev actually approach them to discuss the possibility of using their translation (while offering recognition of their hard work).
So why take the risk of potential making peoples angry / defiant ?
This is all hypothetical as we don't know the in and out of what happen.