While I call that a feature ... some might call that a ... umm ... bug ... 
Edit the file:
/includes/classes/shopping_cart.php
Around lines 1678 - 1681 you should see:
PHP Code:
function actionMultipleAddProduct($goto, $parameters) {
global $messageStack;
while ( list( $key, $val ) = each($_POST['products_id']) ) {
if ($val > 0) {
Change that to read:
function actionMultipleAddProduct($goto, $parameters) {
global $messageStack;
while ( list( $key, $val ) = each($_POST['products_id']) ) {
if ($val > 0) {
$adjust_max = false;
You are just adding the: $adjust_max = false; below that code ...