I would think the problem is that they're highly confident a good number of students cheated, but it's harder to decide on an individual basis.
E.g. if half the students' submissions had an 80% chance of being AI-generated then something is very fishy in the population - perhaps a 99% chance a quarter of the students cheated - but 80% is not really high enough to punish an individual, especially since AI detectors aren't very reliable.
E.g. if half the students' submissions had an 80% chance of being AI-generated then something is very fishy in the population - perhaps a 99% chance a quarter of the students cheated - but 80% is not really high enough to punish an individual, especially since AI detectors aren't very reliable.