"I guess I'm confused, what imports are being brought into this country that are not already here ?"
Uh, it's got nothing to do with the particular species per se, species that are imported are also grown domestically, which does away with the chance of acciendental introduction of disease (as does quarrantine in a certified greenhouse). Restrictions have to do with avoiding what COMES IN ON THAT SPECIES. It can be all sorts of crap --insects, fungus, etc in the soil, in the trunk, in the vascular system. Imported lumber introduced Dutch Elm Disease, Chestnut blight, and other very very bad things came in on plant products in the first part of the 20th century before restrictions were placed.
There is a huge threat coming from Asia with unknown fungal infections, insects and diseases. Asian Longhorn beetle is one of the worst, Introduced infestations have been dealt with severely in the U.S.
http://en.wikipedia.org/wiki/Asian_long-horned_beetle...
It seems I remember my grandma telling me about sixty years ago, something about closing the barn door after the horse has escaped.
keep it green,
Harry