Yep, you can fault them for not understanding it.

Exactly the same as in professional environments: you can use LLMs for your code but you've got to stand behind whatever you submit. You can of course use something like cursor and let it go free, not understanding a thing of the result, or you can step-by-step do changes with AI and try to understand the why.

I believe if teachers relaxed their emotions a bit and adapted their grading system (while also increasing the expected learning outcomes), we would see students who are trained to understand the pitfalls of LLMs and how to maximise getting the most out of them.