The seatbelts serve 2 main purposes:
Purpose 1 - In case of an evacuation, it gives greater control to staff. 'Please don't undo your safety belt', etc.
Purpose 2 - A visual check for guests to know they're secure.
As time has gone on, Purpose 2 has become more redundant as guests generally know they're safe. But 20-odd years ago, seeing a seat belt was a big thing.
Purpose 1 only truly matters when there's people sat in the seat. Plenty of parks around the world don't bother doing seat belts on empty seats. And within Merlin, it's a ride-by-ride thing. This is not an example of H&S standard slipping.
The only thing that's extremely shocking here is the amount of time you must have dedicated to try and find another crazy thing at Thorpe to complain about.