node_load not show taxonomy terms i dont know why

the script node_load(7) output is :

public 'field_tags' =&gt; 
    array (size=1)
      'und' =&gt; 
        array (size=2)
          0 =&gt; 
            array (size=1)
              'tid' =&gt; string '2' (length=1)
          1 =&gt; 
            array (size=1)
              'tid' =&gt; string '3' (length=1)


why the field tags not filled with tag title ! it only shows tid's 

does this is a normal behavior of Drupal node_laod that it only load's primary id's of relational contents like taxonomy terms 
?
Yes, that is the correct and expected behaviour. Loading every entity referenced by a node every time it loads, could potentially get very heavy on the server very quickly. That's why referenced entities are lazy-loaded (albeit manually as far as core is concerned).

To load taxonomy term entities from the tid, you can use taxonomy_term_load(). If you want an (arguably) slightly more pleasant experience when working with entities/fields on code, check out the EntityMetadataWrapper class, which is part of the Entity API module.