A few points:
-Mere possession of viable cannabis seeds is definitely illegal in certain US States where possession of ANY portion of a cannabis plant is against the law. There may be certain jurisdictions where mere possession of seeds isn't illegal by State law, but. . .
-Importation of viable cannabis seeds is against Federal law. Possession of viable cannabis seeds is explicitly against Federal law. This means that there is nowhere in the USA where possession of cannabis seeds is truly "legal". Here is the relevant law from the 1970 controlled substances act:
(16) The term "marihuana" means all parts of the plant Cannabis sativa L., whether growing or not; the seeds thereof; the resin extracted from any part of such plant; and every compound, manufacture, salt, derivative, mixture, or preparation of such plant, its seeds or resin. Such term does not include the mature stalks of such plant, fiber produced from such stalks, oil or cake made from the seeds of such plant, any other compound, manufacture, salt, derivative, mixture, or preparation of such mature stalks (except the resin extracted therefrom), fiber, oil, or cake, or the sterilized seed of such plant which is incapable of germination.
I note that the Federal law specifies
cannabis sativa. So perhaps that leaves a loophole for
cannabis ruderalis (or less likely,
indica) though I wouldn't want to be the one trying to defend that distinction in Federal court with a 5 year felony prison sentence hanging in the balance!
-An attempt to import Cannabis seeds could be construed as intent to cultivate, which is against Federal law, and State law in most States.
-Even if it isn't by itself enough to obtain a criminal conviction, interception of an order for cannabis seeds could be enough to generate "probable cause" for law enforcement to obtain and execute a search warrant to look for other things (including review of your bank records). Anything illegal discovered during execution of a legal search warrant would be fair game as evidence for criminal charges, even if not specified as a target during writing of the original warrant.
-US Customs has the legal right to open and inspect any package entering the USA, and they do, in fact, open and inspect packages every day.
-Despite this, my understanding is that most seeds ordered from overseas go through customs just fine. There are just too many packages coming into the USA every day from all over the planet for every single one to get a complete breakdown and thorough internal inspection. Some number are intercepted, but its the policy of US customs to just issue a strongly worded letter indicating that they have confiscated some contraband, rather than refer individual cases to State or Federal law enforcement for prosecution. In this day and age, I'd imagine they keep some record of this happening, when it happens, but I don't know that for sure.
-In practice the DEA and FBI have bigger fish to fry than to try and make a Federal criminal case on importation of a few seeds. But if for some reason you were otherwise a "person of interest" there is no reason why they couldn't.
-In short, while it definitely is possible to face criminal sanction for importation of seeds, in practice, its not that likely and it doesn't happen that often.