The reason we calculate it like this is because in most countries tax is in full and you shouldn’t be taking a commission of taxes. This is also done on a per product basis. Until I write version 2 of free when commission calculations will be at the order level instead of the product level. I will however continue to treat taxes and shipping separately.
At this time you’d have to custom code your own commission calculator to work like this. Shipping and taxes and the prices are all calculated separately