The amount to which an environmental cause is considered extreme is in direct proportion to the inconvenience it causes others, notably hunters, fishermen, corporate and private property owners. When it starts costing them money or depriving them of hunting or fishing or owning land, it becomes "extreme."
This (and, as you also mentioned, the measures they employ) is probably a true statement. However, to put it another way, this is basically stating that extremism occurs when environmentalist concerns start encroaching on the concerns and rights of others---ie, it is viewed as 'extreme' for those reasons.
For myself, I love the environment--grew up in Montana and Washington, where being outdoors was one of the wonderful facets of life there. As someone who is also conservative, I have always been frustrated by the supposed yin and yang of environmental vs. business and other concerns. I don't think they need to be mutually exclusive. First, business needs to realize that promoting the environment can often be a benefit, not a cost. Second, environmental groups should realize (and many have) that there are frequently business solutions to environmental problems (ie--buy the land, etc). Also, I think environmental groups would be able to accomplish more if they put forth proposals that made business sense (ie, doing this for the environment saves money or increases revenue through.....). One of the other reasons environmentalists are often looked at as 'extreme' is because they seldom seem to put forward compromise proposals: by definition, this makes their position extreme (just as business positions that allow no room for compromise on environmental issues would also be extreme).