I always refuse to take mine off and the TSA barker out front always gives guff, but of course my belt has never set off a metal detector.

I think it's like a lot of things TSA related. There's no regulation, there's no requirement. They just yell at people to take them off, with the ever-present threat of a retaliatory pat down or secondary inspection if you do not comply. It's pure laziness on their part, but then again, a lot of what passengers are forced to do is based on making TSA screeners' lives easier, not the paying customers.