This is very logical and it should be as it has been developed.
Nobody should be able to change a document after its approval. Lets take an example off real world. suppose if you got approval for a loan of 100,000 USD from any bank. Should anybody be allowed to change anything in this approval document???
Now for your issue, there can be certain ways, while you go to configure a PO Workflow, you have got two choices
1) Approve Purchase Order
2) Approve Purchase Order editable
#2 means approver can edit PO, during approval process. example if you applied for 10 days of leaves, your manager may make it 8, or 12 during approval process.
so, you can configure this workflow with #2 option
Also you can manage rules in purchasing policy, you can setup what changes should trigger change request and what changes should be ignored. I am not very good at configuration of purchase polices, but Purchase policy allows you this much flexibility