They're banning hearing aids too. On the basis that you could use them to cheat in approximately the same way that you could with most hearing aids made in the last 40 years (ie. playing a signal into them over Bluetooth or FM or induction or whatever from some hidden device in the immediate area). It probably helps not to be deaf enough to need hearing aids, lest the audio leakage give the game away.
Like most clever exam-cheating methods, it's more effort than revising properly, and only works for the sort of subject that can be usefully aided by a short list of key facts. A smartwatch that you could interact with is likely to be more useful, and won't require you to knit a jumper with binary-coded data in the pattern, train a shitehawk to jump on the roof in morse, or molish a new processor for an exam-legal calculator or anything.
The people who get caught aren't the clever ones, they're the desperate ones who do stupid obvious things like stuff notes down their trousers and go to the loo a lot.