Doing the bare minimum to properly follow rules set by someone else is not unethical. They were doing the bare minimum as outlined by the rules. If that's not enough then the rules need to change.

If I gave you a list of twenty things you can do and I state that you only need to do five of them, are you unethical for only doing five? Now add to that where I pay you money on the first five items but then you pay me money for every item completed after that. Are you unethical for only doing five?

You can say that their efforts at doing the bare minimum were making them money and I would respond that's more the reason to change the rules. A business exists to make money and will follow the rules as best as needed to keep making money. Whether the rules concern civil laws, criminal laws, and tax laws. You can see this throughout the business world, it's not a new concept.