The cap can be lost or broken and someone designed replacement cap (with their logo on it). Retail price of the original part is around 10 $ + shipping, so printing it can save huge percent of that cost.
It shows how home 3d printing can change economics of replacement parts and raises some questions about IP rights. Since I'm sure that this trend of DIY replacement parts will continue I wonder what will be the response from the corporate manufacturers and design IP rights owners.
Files can be downloaded here (I think that the advertisement logo can easily be replaced or removed):
If you don't know what Jawbone Up is, here is a review: