For GET method, parameters are sent as part of the URL (the query string), for POST method parameters are sent as part of the body, that's why in the POST case you don't get the parameters, as they are searched in the body not in the URL.
do I need to manually dissect a query string to process them in cases of a POST action ?
Yes, if you are in the case where you are sending a query string but using method POST, you'll have to parse the query string by yourself, unless you honor the standards and send parameters inside the body rather than in the URL.