I've wondered that a little bit myself. I seem to remember even seeing on Sabco's site that their keggles have internal tri-clamp fittings.
I agree that during the boil, you're effectively heat-sanitizing the clamps, so the risk of contaminating your wort would be minimal. From a sanitary design perspective, the only way I'd consider that to be acceptable would be to remove and clean/sanitize by hand the clamps themselves after the boil/cooling is done. There's no way you can effectively CIP a tri-clamp in place inside a vessel.
At my day job, 3A, FDA, USDA, etc standards wouldn't let you put a tri-clamp inside a process vessel - they're designed to hold together other fittings in a sanitary matter.
I'd argue that anyone who thinks they're getting their kettles truly clean if they have any weldless or threaded fittings on product contact surfaces is kidding themselves unless they're tearing down the parts and manually cleaning them.