The flags "Calculate Tax" and "Determine tax base" are just to make online transactions easier. If it would be set also in background the user has no influence to check if the flags are correct or not. They are only in foreground because of security reasons.
Furthermore it could lead to breakdown situations in batch runs. The effect would be that a lot of jobs would not be finished successfully.
The same information is stated in note 124655, point 2:
2. You cannot carry out tax-relevant postings like telecommunications expenses to bank using the electronic bank statement. If despite this you want to create such postings, you must implement this via the user exit supported (enhancement FEB00001 via Transaction CMOD/SMOD) or by making the corresponding settings in Customizing and carrying out a subsequent processing. In the event that subsequent processing is carried out manually, for example within batch input processing, you may encounter problems with the tax breakdown. In this case consult Note 106271.
In FEBAN in menu 'Edit' -> 'Posting Mode' there are 3 entries:
In Foreground -> XTX will be used
In Background -> XTX will not be used
After Error in Foreground -> XTX will not be used