We need to separate VAT from Sales Tax.
The main reason is that VAT taxe is a very important type of tax in EU (and I guess on some other places) but not in US. An example of problems that arise if we not distinguish them is shown in
OFBIZ-362. There we see that VAT in EU has specific mandatory constraints.
This is a huge work I guess. We will see from futur if it's really needed...