The question is how much of this "limitless, clean electricity" will be created, and how much of it will go into the atmosphere as heat. If you can pump enough heat out of these fusion reactors and the appliances they drive, CO2 induced greenhouse will become irrelevant to global warming. Even if we were to scrub the entire atmosphere clean of greenhouse gases (and stop exhaling in the process), natural atmospheric water vapor would be enough to trap the tremendous excess waste heat of all these fusion reactors. The result would be increased global warming.
A workaround could be to use infrared satellites to monitor infrared heat and report "severe abusers". Every machine could have a waste calorie quota, and if you pass it, the man comes and takes Mr. Fusion away from you.
This is a condensed view of what the informed environmentalists express - at least those with whom I have discussed this topic. I haven't heard all the "wing nut" arguments, but this concern makes sense from a planetary thermodynamics view.
Limitless energy trapped on a planetary surface is a bad thing if you want to keep your biosphere intact.