An actual JSON request would look like this:
data: '{"command":"on"}',
Where you're sending an actual JSON string. For a more general solution, use JSON.stringify() to serialize an object to JSON, like this:
data: JSON.stringify({ "command": "on" }),
To support older browsers that don't have the JSON object, use json2.js which will add it in.
What's currently happening is since you have processData: false, it's basically sending this: ({"command":"on"}).toString() which is [object Object]...what you see in your request.