[danskeren]

API's that charge by usage usually have terms that prevent caching API results. This also applies to Bing:

> You must use results you obtain through the Services only in Internet Search Experiences (as defined in the use and display requirements) and must not cache or copy results. (https://www.microsoft.com/en-us/bing/apis/legal)

It's really unfortunate Microsoft doesn't provide a free API that require users to display ads, or at the very least make their ad network accessible to all.