Greenpeace, simply put, is terrified fusion will work. One of Greenpeace's former head guys has "switched sides", and is now an advocate for the expansion of much "dirtier" fission technology, as he's come to the pragmatic realization that the issue of nuclear waste disposal is much smaller, than all other forms of chemical pollution from fossil fuels combined.
Most of the major environmental org's are just a front for anti-capitalisim, with a hefty dose of
"Think of the planet!" instead of the more tried and true,
"Think of the children!" 
If/when commercial fusion power is availible, there's no upper limit on humanity, at least from a resources standpoint. Direct desalinization of seawater becomes economicaly viable, and almost any recycling technology, whether waste disposal, or materials reclamation, is suddenly economicaly feasable.
They want to hold us back. Even if humanity can "have it's cake and eat it", and enjoy limitless economic expansion, and protect the environment at the same time, they'd fight it. They see themselves as the gaurdians of Earth against the "arrogance of man". etc. etc. etc. And the concern over the "damage" we do is just a facade.
That is never more clear when one realizes their support of "renewable energy" is a sham too. Whenever a major windmill farm or solar project is proposed, there's some environmentalist who invariably springs up to fight that too. They always manage to find some poor bird that'll get chopped up by the blades at night, or some rat that'll be disturbed by setting up the solar panels.
It's really no different than gun control being about "control". At it's roots, the modern environmental movment is the same way.
