Initial idea of mine was the same. Have this as an add-on for the blog module but since I haven't seen any code of the blog module I thought of creating this module as a general one.
Yes we could wrap this as an widget, but the thing is widgets do appear in a sidebar or specified area along with the other widgets. The common pattern for having social bookmarks is just below the post. Anyway I guess it's better to allow the template designer to place this at any place they wish.