For sterility/cleanliness sake, you can really clean your dishes anywhere as long as your technique is good and you use soap and water and/or disinfect. Do it in she shower - 2 birds one stone shit. Mush the food into the grate.
But for the sake of a business trying to keep a license to operate - getting caught cleaning dishes where you shit is a one way ticket to revoked license/failed inspection for a restaurant or cafe or any place that serves /cooks food or beverage.
If you swab the plate and it's clean, IDC if you cleaned it in a portapotty and I'll be fine if I use it. But the health dept will give a shit