But mathematics is itself a science!

It uses a priori truths and deduction from it to arrive at new results.

Additionally several branches of philosophy and logic use a priori facts to derive results from it.

Praxeology is one of different ways of scientific reasoning. Just because you cannot get quantitative results from it, doesn't mean its unscientific. It cannot get you very far in predicting concrete human action, because in its view, humans act based on a subjective value "function" that has a discrete scale (you can only say: A is better than B, not by how much). and since every human is unique with their own values, which are simply not objectively comparable, there is just no way to make predictions or qualitative assumptions from this approach to science.