Bing doesn't make its search index public. It has a query api, where you can provide a search query and get Bing's results on a per-query basis.
The closest comparison to what this person wants is Common Crawl, which is minuscule compared to the big players, and is a 100TB gzipped download that gets updated monthly.
Bing provides API access to its search index. IIRC the type of queries you can do are a bit more sophisticated than just "what's available via the Bing search engine" (e.g. select a market).
That's not index access. You are getting ranked results. Index access would give you the posting list for the term "moose", or the intersection of the posting lists of "moose" and "caribou", or whatever.
You can't make a neutral service that returns ranked results, because that's contradictory.
The closest comparison to what this person wants is Common Crawl, which is minuscule compared to the big players, and is a 100TB gzipped download that gets updated monthly.