They do it to defend you. You've very vulnerable to tigers, wildebeests, and used car salesmen while pooping. Dogs are basically standing guard while you can't watch your surroundings. That's why they look you in the eye when they're pooping. They're watching for you to react to a threat they can't see.

They're being good comrades so you can poop without fear of being eaten by a leopard or a smilodont.