This used to drive me nuts, lol. Max loved sitting up on the back end of the couch against the wall while we watched movies, etc. I picked up the couch pillows a week later and found a whole bunch he'd been dropping the whole time. I was SO glad they're dry, omg.
After that, max got a long time out in his room/living area immediately after i caught him doing this while on the couch (it was really hard to be the disciplinarian mommy and not go into his room and give him snuggles while he was on time out, but it worked!). It took a quite a while, but he did finally get the idea it wasn't ok. He doesn't drop poos while playing on the couch anymore.