So, basically Amazon is in a win-win situation. If the status quo prevails, they're happy. If they have to start charging for shipping, maybe some customers go to a local bookstore but Amazon also has a legal mandate to make more profit in their book sales by not subsidizing shipping out of the wholesale margin. And while some people will buy local a bit more, Amazon sells so many things and the local store may still…
If Amazon has to start charging for shipping, they will be charging the same as every other bookstore in total, making them just another option among the others. Some will still prefer Amazon, but there is no downside anymore to chose another online shop (and there’s plenty big and reputable ones).