Don't Shit Where You Eat

Don't shit where you eat. I'd like to meet the genius who first held forth with this pearl of wisdom. I mean, what the fuck? I don't even think I know what this means.
In a literal sense, it should be self-evident that these two activities aren't conducive to common ground.
If you shit in the kitchen or wherever else you routinely eat, well, that would be a mess. And you could slip on the tile floor and crack your fucking head open. But what if you're eating at a restaurant? Granted, you shouldn't shit on your chair or in the booth or even under the table, but can you not use the bathroom there? If you're having a picnic in the park, you definitely shouldn't shit on the grass or in the sand or even in the water, but you could use a port-a-potty, right? Not that anyone really wants to use one of those fly traps. So you sort of are shitting where you eat. And you definitely shouldn't shit where you eat if you're at a party at somebody's house or something. If you pick up a carrot stick off the vegetable tray, get some dip on it and take a shit on the dining-room rug, well, bad manners.
And if you shouldn't shit where you eat, the converse should be true: don't eat where you shit. First of all, there are practical considerations. A toilet isn't the most comfortable accommodation, especially for having a meal. After a while, the discomfort, especially the pressure on the back of the legs, would likely make the experience less enjoyable. Combine that with having to balance a plate, utensils, napkin and drink, and the challenges multiply. You could use a tray on your lap, but if you have your napkin on the tray, instead of your lap, that could be considered a breach of etiquette. And using toilet paper instead of a napkin is just gauche. And how do you hold your reading material if there's a tray full of food on your lap? The complications are myriad.
All of these situations apply to the port-a-potty, also. Most people I know don't even want to shit in one, let alone eat in it. There's even less room than a regular bathroom, they smell worse and they're sweltering in the summer and cold in the winter.
Now, let's look at this less literally. I've heard "don't shit where you eat" most often in reference to interoffice romances. It might make some sense in that context, if shitting is having the affair and eating is collecting a paycheck. But then why is it not "don't fuck where you make money"? Unless you're a prostitute, of course, in which case you have to fuck where you make money.