Why does my cat keep doing this?

do you discipline him? if not thats why because he knows he can get away with it. next time he does it stick his face close to it so he can sniff it then say no no give him a pat on the butt then don't show him love for 30 min. he'll enventually stop and show him his litter box. put his poop in there let him in the litter box sniff the poop then pet him lovingly and say good boy. hope it works