A few things actually, although none of them being legal problems:
First, if you are a developer, would you trust the translation of your game to a company that is known to steal unprofesional translations (no matter how good some may be) and maybe polish them a bit before inserting them in the game and charge you for the full translation? I wouldn't. Best case scenario, the fans are going to be pissed because they sympatize with the fan that was robbed. Worst case? That, but also add a spotty translations that may also have broken your game in several places, so players dismise your game as garbage because of the translator's less than honorable actions. Sure, the broken mess can still happen even if they didn't, but we are assuming they are profesionals and have a quality testing phase, so it should be less likely.
Second, many dlsite game developers are indies, so it's likely they are closer to the fan side that the corporate side. Again, I'm talking as myself, but if someone where to make a fan-translation of an hypothetical game I made to, say, japanese or english (I'm spanish, so most likely my game would be made in spanish), I wouldn't mind too much, because it would make it more likely for the game to reach more people (sure, many would pirate it, but that's something that IS going to happen anyways, and there would be those that were interested but didn't buy it because of the language, that now will do it and apply that fan-translation). I wouldn't trust a company that is willing to steal, period. Specially if they pretend to charge me for said stealing (at that point, I think I'd actually talk to the fan and make them an offer to polish (if needed) the translation and officially publish it, giving them full credit).
That's from the top of my head. I'm sure I could find more problems if I were to put the effort but at this point this is all just theoretical so I don't feel like doing it xD
In any case, the tl;dr of this is that the loss of trust between developers and a translation company could very well spell the end of said company, or at the very least, a big hit on their business, and stealing something and making it pass as your job is a good way to loss said trust.
