The Wormwood Society has a ton of info on Absinthe including the question as to whether it is illegal in the US:
Is Absinthe legal in the United States?
While there is no law which prohibits absinthe by name, any drink which contains in excess of 10ppm of thujone is prohibited from being imported into, or produced for sale and consumption in, the United States.
NOTE: No regulations have changed. Prior to May 2007 it was not widely known that the acceptable margin of error for official thujone analysisâ€”10ppmâ€”is such that it effectively legalizes many European absinthes. This is a major breakthrough.
Most of the laws which impact absinthe in the US are out-dated, unfair, convoluted, illogical, self-contradictory, un-evenly enforced and misunderstood even by those in charge of enforcing them.
Due to recent changes in the understanding of these regulatory issues, absinthe is once again reappearing legally in the US market. Some of these products are very good representations of what absinthe was a century ago.
Making your own:
Distilling any alcohol in the US without proper authorization is a federal offense (tax evasion) which is enforced by the the Alcohol and Tobacco Tax and Trade Bureau (TTB), not the ATF, and is prosecuted by the IRS. They are quite serious about this. Heavy fines, seizure of property and/or imprisonment may result. We do not encourage this.
Possession of imported absinthe:
Absinthe is not a scheduled controlled substance and is not a drug or narcotic. There are no laws specifically prohibiting possession or consumption of absinthe in particular. Since importation of non-FDA compliant absinthes is prohibited and there are no proper means for ensuring duties or taxes, any non-FDA compliant absinthe in the United States is untaxed liquor, which is illegal. As such, technically, it can be seized during transit or from your home, with a warrant. Seizure in transit is fairly uncommon, but not rare. Seizure from home by warrant is unheard of.
I still don’t quite understand if it is illegal or not.