The primary focus of Instagram is to stimulate original sharing and this is exactly why they have not supported the ‘Regram’ button. However, in a recent development, Instagram is currently testing a feature where you would be able to share other’s posts on your story. As a result, the followers of A would be able to see the post of B if A opts to share B’s post on his story.
The feature is being tested for now with its access to too few people. However, this will be available for all the users sooner than later.
Users (with public accounts) who are not interested in this feature can tap on to the Settings to disable it entirely. The rumored ‘Regram’ button never rolled out. Instead, Instagram has come up with this resharing feature.