Why? Because the assumption that everyone makes here is that the only cause of this damage is from heat/high energy, and that could be wrong. I think the more troubling possible danger (to trees, or humans) is _interference_. Living things are complex systems that use low energy electric signals throughout. If some radiation were able to interfere with one of this processes, even at very low energy level, it could do far more damage than something with higher energy.
I'm not claiming to say any of these threats are real, or the science is good, just that we shouldn't dismiss concerns about X because it's less energy than Y, and Y seems to be safe.
We have to learn more about how the things we're trying to protect work, and particularly what types of radiation we should avoid.
For example, here's a PDF talking about the potential risks of low energy radiation on human cells. http://ec.europa.eu/research/environment/pdf/env_health_proj...
And a longer article about some possible dangers of electromagnetic radiation: http://www2.fiu.edu/~mizrachs/EMF-Hazards.html