Topic: Rails 3 remote form processing as HTML

I'm sure I'm missing something simple. I want to simply update a record in the background without rendering anything or changing the page, but my remote form passes data as HTML, not JS.

In the view:

    <%= form_for event, :remote => true do |f| %>
      <p class="bottom small"><%= :status, Event::STATUSES, { :prompt => 'Mark as:' }, :class => 'status_update' %></p>
    <% end %>

Then, my JQuery to submit form on change:

<script type="text/javascript" charset="utf-8">
  $(function() {
    $('.status_update').live('change', function() {

In my controller:

  def update
    @event = Event.find(params[:id])
    @event.attributes = params[:event]
    parse_time_and_duration if params[:starts]
    respond_to do |format|
      format.json do
        render :json => { :id =>, :title => @event.display_title }

When I do this, I get a missing template error:

Missing template events/update with {:formats=>[:html] ...

Because the controller is trying to process it as HTML, not as Javascript. I thought that was what :remote => true was for? If not, what's the point?

So I found I can get around this by explicitly adding :format => :json to my form_for:

<%= form_for event, :format => :json, :remote => true do |f| %>

But can anyone tell me why I have to do that? What am I missing?