You are mixing very different things, on the one hand there is the one that is legal to make mods, on the other hand there is the copyright. Just like when you buy a car in real life, you can make modifications to it. The same thing happens with games. ETS2 and ATS support workshops, therefore it is completely legal to make modifications to the game. What you mention about incorporating brands of other trucks, that is already a legal problem, because they will have to pay for the rights to use the brand, in the same way that SCS does it by paying Volvo, Scania and others.
No, you went too far ...
Exactly, they make and undo at will, it is their project, their mod and nobody can tell them anything. It is simply an advice that perhaps the community is giving them, since the games that really grow and come forward are those that listen to their community, SCS is a developer who knew how to listen and, within its possibilities, do what the community proposed. Something that marks a lot is that developers communicate with their community, listen to it, not only to know what they want, but to interact with it, share news, improvements, interact with people who publish bugs and more. I didn't really see many posts on this forum, but the few I did see, when someone publishes a TMP bug, they simply move it to a special state, and it is left open for community comments or closed. I didn't see the developers really involved with the community. I do not doubt in the effort that developers are making for this, I also know that it is not easy to fulfill a project like this (as a hobby) and in turn fulfill daily life. It is free, the decision is made by them, the community is generated based on the seriousness and importance that the developers give to the project, when the developers themselves begin to excuse themselves, for example saying "we cannot work without money" and then the community offers a solution to make it paid to the mod, or with subscriptions, and they put another excuse ... That is where the problems begin.
All I'm saying is just my point of view, as a developer and leader of various projects.