Maybe the expression came about, many moons ago, from another expression "eat **** and die." When told to "eat **** and die," perhaps the person took him/her up on the offer to prove that you couldn't actually die from eating **** (alcohol may have been involved). Having survived the feast of crap, he/she just looked at the other with a grin and said, "told ya."
It could have also come from the look on a dog's face when they eat crap, like our Golden. If we catch her, we start carrying on, telling her to quit. She just looks up at us as if to say, "what am I doing wrong...it's not so bad."
