Malc,
Things can become "feral" when they are introduced into a new environment, with no natural restrictions on them. I have read that some species of Eucalyptus trees have become a huge problem in the Florida Everglades, whereas the Australian bush is full of them, but they are a native tree and are limited by climate, soil type, etc. Apparently, there are no such limitations in Florida, and they have gone absolutely wild.
The Australian native possum was introduced into New Zealand many years ago, and are now a massive problem, causing enormous damage to local flora, mainly because they have no natural predators in NZ. I previously mention rabbits and foxes, both of which have caused havoc in Australia.
Oleanders, on the other hand, are a known poisonous plant and, like you, there are no restrictions on the sale of them here.
Peter.