@john-monne (I'm making a new comment here instead of replying above, because I will not get notifications otherwise (it's a forum bug))
You are looking at this only from your personal interest perspective and I can understand that, however I'm sure that if you try to look deeper into this you will understand that both customers and designers have interest to eliminate thieves.
I'll try to explain the whole picture:
1. You are presuming that we are selling hundreds of copies of a model that we create (actually some amateur designers presume that too), but that is extremely far from the true.
My average stats for the last 9 years, roughly show that only around 5% of the models get more than 10 sells (total - that makes ~1 sale per year - for only 5% of the models). Some models never get a single sale.
2. Models do not have eternal life once created - TurboSquid released an analysis for their entire database some years ago and estimated that a 3D model lifetime is around 3 years on average - reasons: the requirements of the industry change, software versions change, etc.etc. I'm sure that gets shorter now.
3. The models price that you buy from these thieves now is that low, just because they do not spend any time in designing and modeling - they just download them from a pirate site and upload - that is all that they do.
They also do not pay for software licenses needed to create those models and that is why they can afford to sell them at that price.
Those who actually spend time to design and model can not compete with this pricing, they will be losing money if they do.
Now I'm sure that you don't care and don't want to know that, you just want to buy a model that you like and cheap if possible, but if this continues in the end there will be only thieves left and noone to create them.
Therefore thieves will also don't have from who to steal except from one another (we are now partly in this stage) and there will be no new models for you to buy.
Those terms and conditions, copyrights and laws that may seem weird and obscure to you are created to prevent this, not to make designers rich.
Unfortunately, designers in the 3D World are not celebrities driving Lamborghini..
Just like in the real world it takes a lot of time, education, talent and a lot of hard work to create something that people will want to buy.