You rarely want two-way binding, so it is not something that is in the core of CanJS. We'd rather have the one-way binding that you always want and allow you an easy way to enable two-way binding. There is a blog post from a little while ago where a form was powered by a bunch of two-way helpers: http://bitovi.com/blog/2013/01/weekly-widget-two-way-mustache-helpers.html
I think these helpers will make their way into CanJS in the form of a plugin shortly.