Concealing your license plate or it not being easily visible. The same reason the darkened plate covers are often illegal and why putting it on the inside of your window is generally frowned upon.

It doesn't really matter if it is legal or not for them to do, I'm pretty sure they'll do it nevertheless. I had a cop pull me over for the light over my plate being out. The cop said he couldn't see my plates when he was right behind me... at night with his headlights turned off (He turned them off while driving behind me, turning them back on right before the lights started).