"Monoids", you keep using that word. I don't think it means what you think it means. You've basically said "use monoids" without a word of elaboration how to interpret ethics, or really anything, in this formalism.

Intuitively I get it, but it doesn't seem all that useful. I'd love to see an example of it in action.

The ACM and the IEEE want to do business with each other. Business partnerships cannot exist unless all parties agree to follow the same ethical system. Unfortunately, ACM and IEEE have different Codes of Ethics. To do business, they must define an ethical system that they both agree upon.

The simplest monoid combines ethical systems by only including codes that appear in both Codes.

So, the ACM+IEEE Code of Ethics can simply be defined as the intersection of the ACM+IEEE Code of Ethics.

Then ACM knows what to expect from IEEE, and vice versa.

If we want AI to be ethical, we must be able to express ethics computationally, and we must be able to formally verify the correctness ethical systems.

So, we should use monoids.