Where exactly do you see that POST isn't supported? I don't see anything in that page that says that. Do you perhaps mean this sample:
if($.request.method === $.net.http.GET) {
// get query parameter named id
var qpId = $.request.parameters.get("id");
// handle request for the given id parameter...
var result = handleRequest(qpId);
// send response
$.response.contentType = "plain/test";
$.response.setBody("result: " + result);
$.response.status = $.net.http.OK;
} else {
// unsupported method
$.response.status = $.net.http.INTERNAL_SERVER_ERROR;
}
If so, that's not saying that POST isn't supported in XSJS in general. Its just demonstrating a service that only expects GET. Therefore this one particular service implementation produces this message for any other request methods.