I stand corrected
The Nature Conservacy is a good counter example, although there have been a few abuses there recently.
The voluntary programs you mention are also good examples, although I'm not sure the leading environmental activists figure in this much.
You make a good point that I didn't define activist very clearly. I was thinking of those who are found in the headlines more often than others, although those who quietly lobby for things like smart power meters and tax breaks would certainly qualify under any reasonable definition of activist.