Well, in dire need you have to do what you have to do to survive. It's true that in real life, the risk of infection would be huge, but it would be annoying to induce the infected buff every time we harvested corpses. Still, it's mainly about appealing the theme of the game. In The Walking Dead, the survivors take the flesh of the zombies apart and smear it all over their bodies to go undetected, it's just common to see such things in zombie related media.
I mean, it's poop in the end. I guess I can agree that if it were animal poop it would feel less weird, but it's still not far fetched, and at least we players know exactly where to find it since it's human. Having to look for animal manure on the ground would be a bit more of a chore, imo.