Initially guests wanted it at Helix, it got complaints that it felt unsafe.
Then with Icon its because the restraints had double redundancy in case of failure, the seatbelts give it a third redundancy if something was to go wrong.
Looking at a few others, Copperhead Strike opened with seatbelts (unsurprisingly, Cedar Fair after all) and DC Rivals Hypercoaster didn't, neither do the majority of the Blue Fire clones, Star Trek or Capitol Bullet Train.
Merlin being Merlin, I can just see the guest perception taking precedent with Hyperia. This is the company that wouldn't batch a station at Mandrill and has ugly queue lines where every new ride has fences over 6 foot tall.