>libya is blue
This probably changed after Gaddafi was killed.
Anyway, I very much agree with you.
This is partly because the "modern left" has had a tradition of intentionally or unintentionally subverting apolitical (at least when it comes to left vs. right) movements.
Environmentalism used to be non-partisan in the US. As an example: Nixon was a republican but he created the EPA, clean air and water act, various animal protection acts, and put the National Environmental Policy Act into law. These actions were either directly proposed by Nixon, or he signed them into law.
Since Leftism has traditionally been viewed as the "counterculture" or rebellious ideology (although in certain circumstances it becomes mainstream -take for example FDR, who might not be truly "leftist", but was one of the most left-leaning presidents in US history), any person that joins a "counterculture" or rebellious movement is more likely to be a leftist. As long as the movement is outside of the mainstream it does not have to be particularly "counterculture" or rebellious to be subverted, and unless it is strictly anti-leftist (it does not mean it has to be right wing) leftists will seek to join the movement. As more leftists join the movement the movement attracts more leftists which attracts more leftists and so on until it becomes of a leftist character in terms of terms of both ideology and members. Probably you already know this. This does not mean that individuals within the new movement cannot fulfill the original purpose of the old movement, rather it just means the movement as a whole takes on different character.
A infamous example of this is "Earth First!". The beginning of the movement started with principles based on conservation biology that opposed unnecessary human disruption of natural ecosystems. During the 90s it took on a strong leftist and anarchist bent:
"After 1987, Earth First! became primarily associated with direct action to prevent logging, building of dams, and other forms of development which may cause destruction of wildlife habitats or the despoliation of wild places. The change in direction attracted many new members to Earth First!, some of whom came from a leftist or anarchist political background or were involved in the counterculture. Dave Foreman has suggested that this led to the introduction of activities such as a "puke-in" at a shopping mall, a flag burning, the heckling of Edward Abbey at the 1987 Earth First! rendezvous, and back-and-forth debates in the Earth First! Journal on topics such as anarchism, which Foreman and other Earth First! members did not wish to be associated. Most of the group's older members, including Dave Foreman, Howie Wolke, Bart Koehler, Christopher Manes, George Wuerthner, and Earth First! Journal editor John Davis, became increasingly uncomfortable with this new direction. This tension reportedly led several of the founders to sever their ties to Earth First! in 1990. Many of them went on to launch the magazine, Wild Earth, as well as the environmental group, the Wildlands Project. On the other hand, Roselle, along with activists such as Judi Bari, welcomed the new direct-action tactics and leftist direction of Earth First!."
I think that for an environmentalist movement in the west to fulfill its original purpose and not stray from it, it must be strictly anti-leftist (but again not necessarily right wing).