That's the rule on the Ozark Trail and the Appalachian Trail, to the best of my recollection. However - many times the ground is just rock surrounded by bits of dirt. Do the best you can and use your best judgement. Remember that everything else dumps it in the woods without burying it, but they're not using TP, either. No one wants to step in it, see it, or see your TP scattered about by the wind. Try to make it so no one will know that you did your business there, and it should be fine.
Here's a cat-hole I dug and used the morning of 2013/08/11, that's about four weeks ago. The red ellipse encircles the cat-hole, just to make sure you can tell where it is:
Let's note a couple things. First, there's no erosion. It would be surprising to find erosion in a well-located cat-hole. Second, though it's been there four weeks, no animals have dug it up. There are 'possums, 'coons, deer, bobcats, pumas, coyotes, dogs, foxes, armadillos, squirrels and rabbits in the area. There's lots of scat in the area. I'm not sure what kind of animal might want scat, but there's plenty on top of the ground everywhere you go. Finally, notice that none of the TP is visible, and the cat-hole itself is pretty inconspicuous. So...the "erosion might expose the TP" and "animals might dig it up" theories are not born out here.
Okay, now about the decay of the TP. This example is a worst-case example. When I'm backpacking, I use very little TP, because I start with leaves and finish with a little TP. But here, I'm on my own land, and did the whole job with TP. That means that I used a lot. Furthermore, it's not biodegradable TP, it's grocery-store TP. It's rained once or twice in the four-week period. Like I said, worse-case example, excepting that it could have been drier. What we see here should be far worse than the typical backpacking cat-hole. Let's dig it up and see:
Notice that the ground is bone-dry, despite the rain. We're in the woods under good cover, so no surprise there. There is some TP left, but not much. It's almost completely decayed. What's left won't last long. The TP is well-rotted before any erosion has taken place and before any animals have dug it up.
If you want to pack yours out, or use rocks or sticks or whatever, that's great. But I know how and where I can properly bury mine, so I'll keep doing that. I'm sorry if that triggers your righteous indignation, but it works if you know what you're doing.