This is true and that is why modding is such a grey area. The fact these companies don't come after modders or ask for their stuff to be removed is not a testament that they allow this, in fact as we have been told ourselves directly: it is more a "turning a blind eye" to it.
If there's one thing we've found throughout the years of Modding, it's that a locked or closed model can still be gotten to (maybe newer software has changed this I don't know). It's why we have implemented different rules over the years that were not there before. And sure you can convert all the models you like, but this community like most on the internet, is a privately run and owned establishment and our rules prohibit these types of mods from being posted and that takes precedence.
You are absolutely right. This is a legal grey area as I said, and it's completely in the rights of companies to go after those who extract and convert their intellectual rights from games by way of modding, but a number of them don't. And those modders who do the work of making the models and elements 'fit for purpose' also have a right of their work not being taken too. It's not a legal or copyright thing, just a sign of respect is all.
As I've already said: you can do what you want with what is available to you and post them as you see fit on the places that allow it, but our stance is, to respect the work of the original, in whatever form it comes about. If you cannot abide by this or don't feel it's right in your opinion then I am sorry to hear that.