based upon america's definition for marijuana, seeds are considered marijuana. however, 'sterile' seeds are absolutely legal. and when a website is advertising the sale as a souvenir the consumer is safe in assuming the seed will be sterile, therefore, from a legal standpoint, absolutely no legal action can be taken against you for possession of ungerminated marijuana seeds. there has to at the least be proof that you had intent on germinating the seed you purchased. otherwise you are in the clear.
why do you think when a seed package is intercepted a search warrant isn't issued on the address? if seeds were actually 100% illegal then that would mean a seizure of a seed shipment would be enough probable cause to obtain a search warrant due to the possibility of an illegal grow operation taking place at the residence. however, since the seeds could have been sterile, and are most definitely being sold as a souvenir, there isn't a judge in this country that would sign a warrant due to the interception of a seed package.
tl;dr seeds are illegal, AND they are legal.