Custom line-item Tax not calculated

Commerce tax is not calculated on programmatically added custom line-items. Is there a reason commerce_tax listens and calculates only product line-items ???

Environment:

We have 3 product variation types (A,B,C), but only 2 product maximum line-items at the time can be present: A+C, B+C, A, B, C are the only possibilities (but that's not the point)
and 5 NON product custom line-item types (D,E,F,G,H) (defined via hook_commerce_line_item_type_info).

We have a custom page where we manage the current order in cart, it has views for product displays with filters, so each product display automatically renders the commerce-add-to-cart form (with ajax properties selectors). Thats ok - works as intended.

This custom page also includes different custom forms for adding our custom line-items. Each custom form is simple: field for quantity, field for unit price. We need different line-item types for distinguish them and for possibile future customizations (more fields for each line-item type).
Submits of these forms creates a line item of type with LABEL, commerce_unit_price and commerce_total already set. No product reference within, i just create them via this code snippet:

$order = commerce_cart_order_load($user->uid);
$order_wrap = entity_metadata_wrapper('commerce_order', $order);
$total = $order_wrap->commerce_order_total->value();
$currCode = $total['currency_code'];

// Creating new line-item:
$line_item = commerce_line_item_new('custom_li_type', $order->order_id);
$line_item_wrapper = entity_metadata_wrapper('commerce_line_item', $line_item);

// Setting the line-item core values:
$line_item_wrapper->quantity = intVal($form_state['values']['custom_li_quantity']);
$line_item_wrapper->line_item_label = 'CUSTOM_LI_COST';
$line_item_wrapper->commerce_total->amount = 0;

// Setting the base unit_price:
$price = floatVal($form_state['values']['custom_li_unit_cost']) * 100;
$line_item_wrapper->commerce_unit_price->amount = $price;
$line_item_wrapper->commerce_unit_price->data = commerce_price_component_add(
    $line_item_wrapper->commerce_unit_price->value(),
    'base_price', array(
        'amount' => $price,
        'currency_code' => $currCode,
        'data' => array(),
    ), TRUE
);
$line_item_wrapper->save();

// Adding to the order line-items array:
$order_wrap->commerce_line_items[] = $line_item_wrapper->value();
$order_wrap->save();

Situation:

Step 1 (no VAT rate)

-----+------------+----------+-----------+
type | unit price | quantity | subtotal  |
-----+------------+----------+-----------+
  A  |     196,00 |      110 |  21560,00 |
  C  |      14,00 |       44 |    616,00 | 
  D  |       5,00 |      100 |    500,00 |
-----+------------+----------+-----------+
                    subtotal |  22676,00 |

Step 2 (adding 10% VAT rate)

-----+------------+----------+-----------+-----------+-----------+
type | unit price | quantity | subtotal  |  VAT 10%  | taxed     |
-----+------------+----------+-----------+-----------+-----------+
  A  |     196,00 |      110 |  21560,00 |   2156,00 |  23716,00 |
  C  |      14,00 |       44 |    616,00 |     61,60 |    677,60 |
  D  |       5,00 |      100 |    500,00 |  !!! 0,00 |  !!! 0,00 | *
-----+------------+----------+-----------+-----------+-----------+
                          taxed subtotal |   2217,60 |  24893,60 | **

* - no tax|vat_10 component on commerce_total
** - which is wrong

Probably during the Commerce Add-to-cart submit action there're some rule invoke calls that somehow binds the product line-item typ to be hooked on calculation of any TAX component applyed to the order, while my code is missing something to do so.

Anyone is there to help?