There's a good scientific reason for not wanting tourists carrying seeds to Antarctica - because it spoils research into how nature colonises formerly barren areas.
A good comparison is with the Icelandic island of Surtsey which was formed between 1963 and 1967. The island and its surrounding waters are closed to the public and can only be accessed by accredited scientists who are performing a long term study of how the island is being colonised from a bare rock to one that now holds hundreds of different species.
Even then they have to take care about accidentally introducing new species, in one case a tomato plant sprouted on Surtsey; it had from a seed that had escaped from someone's lunch. Even though there was almost no risk of the plant fruiting in the shall we say - brisk - Icelandic summer, the plant was torn up and removed from Surtsey to avoid contamination.