APINV Workflow approval is set to 'Not Required' if only line types are 'Tax', 'Freight'

 HI All,
Customer is using standard oracle approval process to approve the invoices. If  customer created invoice with item type distribution line, system is showing approval is required. If it's created invoice with tax & freight type of distribution line only system is not requiring for approval. It's showing approval status is "Not required". Customer is expecting, even it's tax or freight type line distribution also, system should check for approval. What is the functionality, is it system behaving as per standard functionality or not?

Regards
Avaneesh