I've got a changeset in code review to make the default redirect resolution behavior of action=opensearch consistent between JSON and XML-mode requests.
Originally the JSON version did not resolve redirects as it was easier to implement that way, while the XML implementation was separately written and did resolution in order to improve display of content snippets. Content snippets are now included in JSON as well as XML since the implementations were merged, and in general it seems to be better behavior to be consistent across data formats for the same requests.
The original behavior can be obtained by supplying an explicit 'redirects=return' parameter to the API request.