This subject does the rounds now and again. A few weeks ago I flew DELTA (not by choice but due to the Air France industrial action). Although the belt sign didn't stay on it certainly came on and off like a yo-yo at the slightest little bump, with of course the accompanying PA announcement to wake everyone up! In contrast, returning with KLM they remained off during the entire cruise. Although the turbulence we encountered was light, most of it was a lot more bumpier than anything we encountered on the Delta flight. The answer is indeed as JFP suggests: paranoia induced by the litigation culture in the USA.