The problem is that non-native species, planted with the best of intentions, can become invasive and choke out native populations. See for example the Buddleia, which in the UK was often lauded as 'the butterfly bush' and which is now a massive nuisance across the nations railways and wildlife reserves.
I have Buddleia in my garden (which otherwise has a native focus). It's not at all invasive where I am, in upstate NY, probably due to the cold winters. In a colder winter it dies back to the roots and may not survive at all.
Now, purple loosestrife is still all over, despite the biocontrol. And don't get me started about Phragmites or Alliaria petiolata (garlic mustard).
