Since

```
Mage::app()->getWebsite(true)->getDefaultStore()->getRootCategoryId();

```

returns `'2'` I've wrote

```
$productCatalog = Mage::getModel('catalog/product')->load($product->getId());
$productCatalog->setCategoryIds('2')->save();

```

but it doesn't change product's category - in Categories tab checkbox "Default Category" isn't ticked.