Even though this is barely anything new when it comes to this fandom (while more so apparent specifically in the MMD community) something that irks me a lot is when people lack the understanding that just because something is free that it is "less illegal" or "not illegal at all" to do various things. So basically I am here today to discuss the issue of why it's illegal to do things such as edit models or extract voices from games despite the fact they are distributed for free.
(Disclaimer : I am no legal expert so I can not gurantee that I am 100% correct on my perspective of laws, I only know enough to understand the gist of it. So do be wary that I may make mis-interpretations on what's being said)
(Disclaimer 2 : I talk about editting games a lot in here and I want to clarify that I AM NOT TALKING ABOUT MODS. Mods are considered to be modifications to that game that's external to the game's coding while editting games in this context refers to hacking into the game to change various aspects of it)
1. It's not wholefully yours
Essentially when you are purchasing (yes, even if it's free it still counts as a purchase) you are agreeing to a set of terms and conditions provided by the sales person on what exactly you are purchasing. When you're buying something you're not actually buying the whole product, you're actually buying rights to use the product in the ways that was agreed upon by the company. Take a game for example, when you're purchasing games you are essentially buying the right to own a physicall copy of the game (if it is a physical purchase), to play the game, and to re-sell the game. With me so far? Now you may be wondering why such a system exists in the first place? It's simple; it's to prevent people from essentially taking the game, perhaps change it in a few places and re-selling it. Or rather, it allows companies to claim property theft when someone attempts to do that. If you're a simple developer who may actually want people to take your game and be inspired to make their own thing with it this isn't so much of an issue but imagine you're a big company who's invested millions into developing a game you wouldn't exactly want people trying to make money of a game that was mostly yours with only a few changes to the game.
Now let's take a break here to talk about free games specifically here because you may be wondering now why it's such a big deal for people who develop free games? Again this is a simple answer; there is no difference between purchasable games and free games. As mentioned before even if you are getting the game for free you are still bound by a contract between you and the company to use the game on the terms they give you. In other words whether it's free or not is irrelevant to the fact that you essentially did not get the whole game, but rather a set of terms to use that game in specific way that's defined clearly in the ToS by the company.
2. You were not given permission to do that
I talk about this idea about terms set by the company to allow you to use things you purchase from them in a certian way but I held back above so I can talk about it more specifically here, even though a lot of it may seem like a re-hash on what has been said above as a result.
But either way, yes you are given a set of terms on how you are allowed to use your game but what if you don't abide by those terms? What if you wanted to in fact edit the game so that, for example, you could make the player run faster, or even extract code or models from the game, but the company made no mention of you being able to do that? Then you just performed an illegal act right there, period. It does not matter for what intent you have when doing these things the simple fact of the matter is that you made no agreement with the company to be able to do those things to something we established was not wholefully yours.
"But they didn't say no!" Do you not know that when someone gives a lack of consent that it is common decency to assume no? With that line of logic I am allowed to hit you under the notion that "you did not tell me that I shouldn't hit you". So let's just get that line of logic out of the way.
Back to point we have established that :
1. When you buy a product you are buying the rights to use a product defined by the company who makes the product.
2. You are only allowed to use the product in those ways.
3. If you decide to use the product in any other way you are breaching the contract made by you and the company and thus are under illegal grounds.
4. It does not matter if the product is free or if you are not doing it for profit you are still breaching those terms.
5. If the company does not state if you can then it means you're not allowed.
I hope I explained this well enough.
EDIT : I realised I briefly mentioned MMD community although never made it clear how it still relates. There's not much to say here other than you are making an agreement with the MMD modeller to use the model under their terms. In other words it's the same thing as all of the above only replaces games with models and company with modeller (Unless the model is the company's work ofc).