A celebrity is often a good spokesperson for public service announcements because their fame gives them a louder voice that people will often listen to more than just random person they don't know. The celebrity can be especially powerful at giving a message if they have real-life experiences with the situation they're discussing.
And it's even funnier when someone does a parody of such a PSA. Who better to do a PSA about gun safety than Plaxico Burress. (Don't know who he is? Then Google is your friend.)