I can give why I agree with Mises, but to understand the problem I think the best place to start might be something like the wiki page on the Is-Ought Problem.
Now, there are various mechanisms people have used to attempt to bridge this gap. In libertarian circles, Ayn Rand’s Objectivism is one. More recently, Stephan Molyneux is attempting something similar.
However, I personally reject all attempts to do so, because I believe the realm of the normative arises in human action. In other words, choosing an end is the source of “should/shouldn’t” our science tells us how things work, our subjectively determined ends tell us what we want. Should arises from an End sought, and until an End is preferred over and against other potential ends, should doesn’t exist.
I believe however, that there is a prescriptive science of the normative. I would call that science Ethics. 
In other words, there is a scientific field that can make IS statements about how and why a man arrives at should/shouldn’t. Ethics is also IMO muddy, because it doesn’t recognize this dichotomy.
The proper hierarchy of philosophy is Metaphysics → Epistemology → Ethics (normative in it’s tech, value-free in it’s analysis) → Politics (normative in tech, value-free in theory).
Acting man appears is a product of the metaphysical realm. But he interprets reality from the Epistemological realm, in acting he must choose, and uses knowledge to do so, what he values (as demonstrated through action produces the field of ethics), and how he manages balancing his values and actions in concert with other human beings is the field of politics. The action interface back to the metaphysical realm is the realm of the physical sciences. It’s theory is an epistemological construct, and which technological products he produces and the ends at which he aims is are discussed at the individual level in the science of Ethics, and in the social realm via a science of Praxeology (economics and politics).
That’s the way I organize these various fields in my head. I think the division between the fields of knowledge is interesting in one way, but it’s all a product of the epistemological layer (what do we know, how do we know, what does it mean to be true). A diagram would probably make it more clear.
Because all of our science could be accurately described as an Epistemologically generated artifact of man’s interaction with the Metaphysical realm (reality). This generated artifact is itself a product of the physical realm. (Epistemology is the recursive science.)