To have a learning experience.
A DM candidate is a student and anything they do should be under the direct supervision of the instructor. They are not in charge of anything. Thus, good luck proving negligence as a DM candidate does not have any duty of care towards others. Insurance doesn't cover intentional torts so I just don't see what you get for paying them as a student. Maybe other countries are different.
It's ok to just say you were wrong
I'm not sure how they do it in other countries either but with the litigious state of America I'd rather have insurance than not have it.
As I mentioned earlier, to be and Active NAUI DM you must carry insurance and need proof of it for the yearly renewal so I really don't see the issue in having it as as candidate as you are going to have to have it anyway to do the job.