The reasoning behind that statement applies on more than one level.
Reason number one is relatively self-evident. Activism is designed to protect, defend, and conserve.
For our intents and purposes here, we'll use the term "animal rights activism."
When we defend the right to healthy existence of an eco-system, we are actually defending the rights to exist for the fauna that resides within that eco-system.
From my personal vantage point, that would define an "environmental activist" as an "animal rights activist" as well.
Let's call it "win-win" for all life impacted by the actions of the activism.
An example of this premise comes to mind, and that is the recovery of the American Bald Eagle.
The history of the plight of the Bald Eagle
By banning DDT, which was proven to be the environmental toxin that thinned the Eagle's eggshells or rendered them sterile, the Eagle population began to rebound.
We also learned that DDT affected far more than the Raptors.
The Bald Eagle's recovery wasn't a casual, "wait and see," occurrence -- it has been attributed to the activism of Rachel Carlson, an American biologist and author of Silent Spring, published in 1962, and is credited with being the signature event in the birth of the environmental movement.
So, I will reiterate that every day is a good day to be an activist.