Here is an analogy.
If a I have a flashlight that puts out a million watts, it would burn you if you stood a foot in front of it right? Now what if you were standing a mile away, you would be receiving much less power and you would be perfectly safe. But the light energy doesn't just disappear, it is spread across a large area. Now, if you build a large collector several miles wide you would absorb all that light. Assuming nothing in between the flashlight and the receiver to block/absorb energy, then you just transmitted 1 million watts a distance of 1 mile to a receiver and it is perfectly safe to stand in the beam because the energy density is so low.
This SPS works the same way. The receiving power density would be less the limits set by the government for exposure to electromagnetic radiation. A large rectenna would collect all that energy over a wide area at >85% efficiency. Walking in the low power density beam is harmless.
Exposure limits: http://www.astrosurf.com/luxorion/Radio/table-power-field.gif
Notice the limit is lower at about the point that a half wavelength is about the same height as a human...