I have an unease about pre-emptive seizure, it is too easy to abuse, and there are reports that it has been.
That said, I have to weigh in on the 10th Amendment argument. The Amendment basically says that if it ain't in the Constitution, then the function is reserved to the states and the people.
I will have to offer there are two distinct places in the Constitution that cover the WOD quite well. The commerce clause, and the common defence clause.
The notion the matter is the province of the several states would mean the problem is located solely within the states. The moment the material begins to transgress borders of more than a couple of states, then the issue becomes one of interstate commerce, and that is clearly under the province of Congress in Article I.
Similarly, when the material transgresses the borders of the country, the common defence clause kicks in if the item is considered a threat to the United States.
Now, that could be the matter of some debate, and prior to September 11, 2001, an argument could be made that it was not a National Defense issue, but rather something of a criminal nature.
Since that fateful day, however, we have come to learn that one of the funding sources for Al Qa'eda and other terrorist organizations is from the sale of coccaine and other illegal substances (heroin way up on the list there). We are at war against terrorist organizations of global reach, so by extension if they are getting some of their funding from "drug money," then it becomes a matter of National Security import, thus satisfying the common defense clause.
These form, in my mind, the two fundamentals on which the Federal Government can indeed involve themselves in the WOD. There is a third, somewhat more tenuous but still cogent point to be made, however, and it has to do with who is paying the bill.
Regardless of whether drug adiction leads to more crime (I tend to think it does, but there are arguments on the other side that say these people are too "bombed" to coherently put together such an operation, so I'll leave that alone), there is a cost to society that is borne because people adicted to drugs do not generally perform functions that add to the value of society (call it "work"), but rather have to receive the generousity of society (call it "welfare").
The welfare that is granted to drug users comes in many forms - unemployment checks, AFDC, WIC, methodone clinics, emergency hospital treatment performed gratis, clean up of our parks and common areas, and so on. Who pays for these? The taxpayer. And it is not just the taxpayers of the several states as citizens of states, but also in their capacity as taxpayers to the United States of America. I, for one, believe that if the US Government is paying the bill, then it ought to use all legal means at its disposal to ensure it is reducing the amount it has to pay.
But the fact it is paying the bill at all produces an interesting conundrum. Proponents of drug legalization argue on the state's rights issue, among other points, yet often (don't know whether you're in that crowd, you didn't say) insist the Federal Government provide money for drug treatment and education. You can't have it both ways. If the problem is a state problem, then it is inappropriate for the Federal Government to be involved in paying out money. OTOH, if the Federal Government is paying out money and that is acceptable, then the threshold has already been crossed - Federal involvement is all right.
Anyhow, that's my opinion.