It's an easy way to abstract away that bit of the tutorial. Rails makes it absolutely trivial for the purpose of setting up an Ember.js example if your focus isn't on the backend itself.
Rails scaffolding creates RESTful resources by default (and Ember.js plays nicely with it too). Starting from a "blank slate" Ubuntu server install, you could have a dev server providing this groundwork in about 7 commands in under 5 minutes. If you were to do this in J2EE (nothing is stopping you), setting up the backend would take significantly longer than the frontend ideas being explained.