Firstly, I want to apologize for the change in bang without notice. We do our best to not change bangs too often as it can break existing workflows, like in your case, and cause stress.
That being said, Amazon does have several bangs (including !a, see https://kbe.smaertness.net/search?q=amazon.com) while Apple Maps had comparatively little, especially given the pattern of !gm -> Google Maps, !bm -> Bing Maps, etc. Any custom bangs you create will override the default bangs, if you want to revert this change for yourself.
