[colonwqbang]

This is akin to saying that socialism is about promoting justice, and who isn't for justice? So how can there be people who say they aren't socialists?

People may agree in principle on what the end goal is, but disagree about the method to get there. Today the "environmentalist" is a political label like "liberal", "feminist" etc. It has a simple surface meaning but also comes with a lot of connotations and baggage, so that not everyone is comfortable pinning it on themselves.