Unable to get columnDefs.render to work on columns coming from AJAX request

Brief background: I need to be able to create a DataTable with a set of columns that comes from a configuration elsewhere in the system. In order to dynamically create the DataTables table I have piggybacked the DataTable init on the return from the "get columns" AJAX request. Up to this point everything is working.

The problem occurs when I attempt to add display rendering. It is not applied. In fact, the rendering functions are never called leading me to believe that it is unable to locate them. I use the columnDefs parameter on DataTables init to attempt to apply the display formatting based on class. Below is a simplified use case:

HTML:

<table id="adjustmentRequests" class="stripe"></table>

Javascript:

$.ajax( {
    url: "getDisplayColumns",
    type: "GET",
    "data": {
        "dataStoreName": "test"
    }
}).done( function( displayColumns ) {
    var requestTable = $( '#adjustmentRequests' ).dataTable( {
        "jQueryUI": true,
        "paging": false,
        "searching": false,
        "info": false,
        "processing": true,
        "ajax": {
            "url": "getData",
            "data": {
                "itemId": "1234"
            }
        },
        "columns": displayColumns,
        "columnDefs": [
            { // Date columns
                "targets": "date",
                "render": function( data, type, row ) {
                    if( data == "1/1/1900" ) {
                        return null;
                    }

                    return data;
                }
            },
            { // Money columns
                "targets": "money",
                "render": function( data, type, row ) {
                    var className = new Number( data ) > 0 ? "positiveAmount" : "negativeAmount";
                    return "<span class='" + className + "'>" + data + "</span>";
                }
            }
        ],
        "language": { "emptyTable": "<span style='font-style:italic'>No adjustments requested.</span>" }
    });
});

Example response from getDisplayColumns:

[
{"name":"Code","orderable":true,"searchable":true,"title":"Code","visible":true},
{"className":"money","name":"Amount","orderable":true,"searchable":true,"title":"Amount","type":"num","visible":true},
{"className":"date","name":"Requested On","orderable":true,"searchable":true,"title":"Requested On","type":"date","visible":true}
]

Example response from getData:

[
[42, 23410000.00, 02/25/2015],
[A1134, -42.70, 02/24/2015]
]

Why is DataTables unable to perform the format rendering? Does it have to do with the columns coming from AJAX instead of the DOM? Have I missed something else?

Any assistance would be appreciated.