More to it than that. Theoretically, all seeds that may be imported have to go through a process of verification / etc. to ensure that the seeds are biologically 'clean' (i.e. do not import diseases unknown in the destination country), are on an approved list of seeds to be imported (i.e. with USDA/etc. approval in the USA), bonded, etc. Vendors can ensure that (they have the scale and means to ensure that seeds are clean, that the paperwork is OK, etc.) while private folk jetting around do not.
For example, the US Agriculture industry is saving billions of $$$ every year because they do not have hoof and claw disease to deal with and hence do not have to vaccinate for it. Similarly, mad cow disease is virtually unknown and also does not have to be tested for regularly, etc. Some countries like NZ are even 'cleaner' than the US and hence preserve that status by having very stringent inspections at border crossings (i.e. no dirt on shoes, disinfecting baths for dirty shoes and all that). It only takes one lugnut to destroy that status and cause widespread costs as herds are innoculated or preventively slaughtered.
So I understand why statutes are what they are. Also consider how often fauna and flora inadvertently finds its way into broader nature, i.e. the Burmese python infestation in the FL everglades, longhorn beetles in the NE of the USA, kudzu in the SE of the USA, the cane toad in Australia, introduced rats throughout the world leading to massive declines in bird populations, etc.