DataTable jquery tr & td selectors not firing

I had asked this question originally on stackoverflow:
( http://stackoverflow.com/questions/26434632/datatable-jquery-selectors-not-firing?noredirect=1#comment41514233_26434632 )

But thought someone here may provide better answer in context of DataTables. I am using version 1.10.2

I have following code to handle clicks on row or individual cells.

$(document).ready(function() {
    var JSON_URL = '{% url "technician_activity" %}';
    var oTable = $('#technician_activity').dataTable( {
        "processing": true,
        "serverSide": true,
        "ajax": JSON_URL ,
        "jQueryUI":       true
    } );
    alert("Without this alert selectors don't work? oTable = " + oTable);
    oTable.$('tr').click( function () {
       var data = oTable.fnGetData( this );
       alert("Column " + data);
    });
    oTable.$('td').click( function () {
       var data = oTable.fnGetData( this );
       alert("Cell " + data);
    });

});

One thing that puzzels me is without the first alert statement

alert("Without this alert selectors don't work? oTable = " + oTable);

selectors for tr and td don't work this is very puzzling to me -- what is the difference that this alert() is making?