"Probably the same theory for how a microwave cooks food."

The actual change to look at isn't really the heating, but rather that RF has been known to increase glucose metabolism in affected cells. My own theory is that if that is true, you're essentially increasing cell aging and oxidative stress with the increased metabolism. Who knows if it's true. The effect seems small and the exposures need to be longterm.

Surely power lines running directly across a residential property represent a long-term exposure.

(I personally doubt there is a significant risk, but if there is, these powerline easements would be quite risky to the residents in terms of ongoing/continuous exposure.)