The issue (if we may call it like that) seems to be restricted to the Lumia 930 and Lumia 630, both of which are brand new handsets that run Windows Phone 8.1. Some users who aren’t based in the UK report that they have the option enabled anyway, while older devices that are currently being updated to WP 8.1 should not encounter this problem at all.
Microsoft has yet to provide an official statement on the matter. Perhaps the company is just looking to promote Bing in a more aggressive manner. We’ll let you know if we hear new details on this.
P.S.: Android handsets allow users to change the default search engine (from Google's) to Bing, Yahoo, or Ask.
sources: The Verge, Tom Warren (Twitter)