In Mage/Review/Model/Resource/Review there exists the function _afterSave which seems to set the customer ID after saving the model. Thus overwriting the changes made before saving it.
I decided to solve my problem by creating a helper function to directly write the customer ID to the table review_detail:
public function saveCustomerIdToReview($reviewId,$customerId){
$resource=Mage::getSingleton('core/resource');
$writeConnection=$resource->getConnection('core_write');
$tableName = $resource->getTableName('review/review_detail');
$query = "Update {$tableName} SET customer_id = '{$customerId}' WHERE review_id = " . (int)$reviewId;
try{
$writeConnection->query($query);
} catch (Exception $ex){
$this->logerror($ex->getMessage());
}
}
One other solution would have been rewriting the _afterSave method in Mage_Review_Model_Resource_Review.