Hi there,
It is necessary to calculate discounts on a per-line basis in order to correctly account for tax. A single invoice can contain multiple items some of which may be taxed and some may not.
Without this, discounts for multiple line items would be lumped together, and all marked as taxable, even though the items being discounted may not all be taxable. This skews the tax calculation, effectively untaxing items that weren't taxed to begin with.
As a result we aren't currently considering changing this behaviour.