The ruling really amounts to the result of the hearing. So ultimately we're talking about the reasoning behind the verdict which has already concluded. Therefore, it would make more sense to talk about it in past conditional terms, as this scenario isn't being considered now that reading an article online makes thieves of internet users.
Now, if the ruling were in progress, perhaps I might be more inclined to say "would make". It is always conditional, but it is the case being discussed in this particular moment, so it wouldn't be in the past tense.
So that said, I would argue that this is more correct:
According to the ruling, merely reading an article online would have
made thieves of most Internet users.
This is very subtle, and I want to stress that because I don't mean to say that this is the absolute correct way. I only mean to say that this is more correct in my humble opinion, but both can be used.