The gas cap on my last truck has a thing that ratchets when it's tight enough. They could invent a mechanism to prevent over-tightening... I think myself that it's an oversight. or something that needs special tools to service...
I would like to see a tensioner, but I can offer a guess as to why they didn't add one from the factory. With the belt's carbon reinforcement, over-tensioning could be as damaging or worse than a belt that stretches a little bit. It also adds a little weight that Polaris was obviously on a witch hunt for with the '13 Pro.