Component Subexpressions one-way by default?

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  val1: 8,
  val2: 8
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<h2>This one works</h2>
{{component (component 'x-thing' val=(mut val1))}}
{{val1}}

<h2>This one doesn't</h2>
{{component (component 'x-thing' val=val2)}}
{{val2}}
<br>
<br>

twiddle.json

{
  "version": "0.6.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember-template-compiler.js"
  }
}

x-thing.template.hbs

<button {{action (action (mut val) 10)}}>Change to 10</button>