"I have seen many feral cats that aren't bashful about that at all, and they might throw a thin layer of dirt on it if the ground is not too hard."

I've witnessed free-roaming cats who bury their own feces, cats who don't bury their feces, even cats who bury another cat's feces--I kid you not. That being said, cats are in general, are much cleaner than dogs as domesticated cats need nothing more than a litter box and kitty litter to do their business. Cats, unlike dogs, don't need to get bathed. 

"The scale of the problem is greater than most people realize, which is why TNR, even if it was seriously implemented, would be not enough to deal with it."

While I'm not blind to the fact that this problem has become so overwhelming, it's still tragic that innocent cats have to pay the price because of humans who have acted irresponsibly. If only those who have abandoned their unneutered/unspayed cats would realize how much suffering could have been avoided had they acted responsibly.