Is it simply because policing powers were never delegated to the Feds in the Constitution, everything else left to the states, the 9th and 10th?
Or is there more to it?
Yes, and that's more than sufficient. The Constitution is explicitly enumerative:
"The enumeration in the Constitution of certain rights shall not be construed to deny or disparage others retained by the people." (9A) -- We have hella rights, and only a few examples are given in the Constitution in order to more explicitly secure them, as well as to give the general idea of all the other rights we have (not rights to one another's property, for example, just rights to be left alone by tyrants).
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." (10A) -- Along with the essentially infinite list of unenumerated rights that we have by birth, we also have powers and authorities, these being rights which we can alienate (abandon) via delegation. For example, if I assign my estate to you via power-of-attorney, I have delegated the authority of managing my estate to you. There is an infinite list of such powers. The enumeration of the powers in the Constitution that the Federal government
does have does not indicate it possesses any other powers... all
other powers than those enumerated in the Constitution belong (by default) to the States or to WTP.
Think of it like set theory. We have some set X of rights/powers. The Constitution names a few of those rights that we have, and this naming shall not be construed to suggest we have only those rights, or anything less than the full set of X rights. Conversely, the US Government is delegated a set Y of powers by the Constitution (from the States or WTP) and this delegation does not indicate that it has any
other powers than these. Note that Y is strictly a
SUBSET of X, a point frequently neglected in discussions of constitutional law -- all the powers the Federal government has originally belong to
us (WTP), and we merely delegated them to the Federal government so that it can perform the duties we assigned to it in the Constitution. That's the contract: here are some duties that you must perform and, along with these duties, we are delegating a finite list of powers to you so that you can do those duties. It could not be simpler. But tyranny doesn't work with simplicitly, so we gotta make it seem so complicated!!