It could be. But then if we want to be so strict, also a model 
<code>app.Todo = Backbone.Model.extend({..</code>
should be <code>app.TodoModel</code>. I don't think we need to be so strict.
So calling single model as <code>Todo</code> and collections of models as <code>Todos</code> seemed to me a good point. But it'is only one approach.