question

It is counter intuitive but the reason is sound.

The meter is capable of measuring 100A in the line conductor, and only measures current in the line conductor. The neutral is only required to provide power for the operation of the meter. The operating electronics are very low current and made up of very small conductors. If something went wrong inside the meter, the only current limiting device would be the one upstream of the meter and this could be 100A, it could be that the DNO cutout is the first upstream current limiting device. The meter electronics would explode and burst into flames and the upstream circuit protection might not even be activated.

So the solution is to fuse the neutral conductor with a much more appropriately sized protection device. If a fault develops inside the meter the small fuse will open and the meter's electronics will be disconnected. Sure, it will still be live, connected to the line conductor and probably still happily passing the current in the line conductor, but it would not be on fire.

The meter remaining live after a fault is not a problem as it is not a final circuit and should be housed in and appropriate cabinet (consumer unit) where other devices also remain live after performing their circuit protective functions.

To be honest, I thought it was really bizarre and not sure what type of fuse you'd use for this either. Mine uses a 1.5mm² unfused wire to the common neutral in the consumer unit. As @WKirby mentioned, it's only protecting the circuitry with the meter.