While I agree with most of what you have said I dont think it is quite as black and white and to clarify my post was mainly addressed towards the collecting of plants / seeds within a country (and not from protected areas either), although this may not have come through clearly in what I wrote.
Specifically I was thinking when writing the post about times in Brazil when I have collected the seeds of plants such as the Lobeira (“The apple of the wolf”) and others from Cerrado fragments outside of protected areas in the garden of my girlfriend’s family and how much joy it brings to watch these plants grow .
In terms of international travel , sure , it stands to reason that there are inherent ecological risks and important legal considerations to take but what about regional domestic travel between federal borders or even within a state?
Plus there are plenty of situations where the seeds of a plant or a plant itself may be purchased/ traded/collected legally from seed exchange groups , websites or shops. The USDA cracks down on illegal plant trades and also heavily monitors and advices seed exchange groups online on these matters.
Moreover, I was not referring to the illegal collection of species like cacti, orchids , cycads. This is monitored by CITES , enforced by law (albeit not as effectively as would wish) and is relatively well publicised by the media (Not as thoroughly as the illegal wildlife trade in fauna though).
Finally it is worth mentioning that there are cases of climate threatened endangered plants ( and yes , I know the practice is still highly controversial and about the lack of scientific consensus to its merits). Moving these outside of their natural range through assisted translocation (yes, of course , with scientific oversight) is actually key to conservation like for example the Torreya taxifolia, coastal red wood, and Joshua trees.