I've never seen a B&M that uses the traditional OTSR without seatbelts, but that doesn't change the fact they're functionally useless. I do remember a video on YouTube though of Nikki from Big Brother riding Air, and she complains regarding the fact it "doesn't have a seatbelt"
As for the Smiler, when I was at IAAPA I asked Gerst this very question as they were pretty much advertising the lap bars as one of the USP's on the infinity coaster. As you would expect, they told me that they tried to convince Merlin to go with the lap bars but Merlin repeatedly said no. This still doesn't answer WHY Merlin decided against it, be it health and safety, Guest confidence, throughput conspiracies etc etc, but at least it confirms where the decision came from.
My guess - it's probably the second reason. We know what Merlin are like for saying "our Market research shows the Great British Unwashed General Public think x is unsafe"