Some Physics I Don't Quite Get

From the APOD:

Alternatively, surface particles may become electrically charged by the Sun, levitate in the microgravity field, and move to fill in craters.

I understand the Poynting-Robertson effect (applied here to asteroid dust instead of free particles but it's the relavent physics), and the whole suspended dust thing. The two-part question I don't know the answer to is a) if the gravity is that weak, why don't the particles just fly off all together and b) why don't we see this anywhere else?

It's a mystery.