That is because you are doing a POST. a POST in REST means you are creating a new object. You are not, you want to GET the List. So you have to do a GET in order for it to work.
In a POST, the accept header is not requesting any type/format back, the response should only have the success status set and the URI to then GET what just got created. So the Body of a POST is always empty.
a GET is the only RequestMethod type to return data in the Body.