Still not quite buying all of it. Correct me if I am wrong. Frost heave is caused by freezing water in the ground. (Ice expands as it freezes.) Agreed? This is why footings are placed below the frost line so that the expanding/frozen ground which causes heaving is above the bottom of the footing. Agreed? I would guess the posts that you see heaving are due to the cement being above frost line not because they used cement to the top of the post. You state that frost heave is not affected by weight. Which is true. Why then would dirt on top of the cement stop the heaving? A relatively thin layer of frozen dirt will not stop additional heaving as the ground below becomes frozen. If that was true, then wouldn't any frozen layer of ground over anything below it stop the ground from heaving.
I will agree with you, if the top of the hole is conical (top wider than the bottom), the problem of heaving is greater.
Most of the time, when I see cement used on posts is for a few reasons. One, the cement creates a larger surface area and thus makes the post less prone to leaning, for example a corner post like you stated. Second, some people dont want to put the post deep enough in the ground to anchor it solidly. I see lots of 6' high wooden yard fences put in with 8' posts. The builder doesn't want the expense of a 9-10' post to put in the ground. A third reason, is cement, if premixed, goes in the hole easy, requires less physical work than tamping.
Ok, last try,
The ground freezes from the top down. Yes, the relatively thin layer of dirt on top does provide a barrier to help stop the heave. Certainly this barrier is more protection than having
nothing provides.
The fact that the concrete is wider when run up too the top, as it always is, from simply digging or boring a hole, as loose topsoil, cave ins, and wobble, even if slightly, sets up the perfect conditions for the frost too tighten around the post, and squeeze it out of the ground.
Yes, if a post was concreted all the way down below the frost line, (about 4' here), and the top was not wider than the bottom, it would not heave, even with the concrete all the way too the top. However, short of using Sono tubes as forms on posts, (not practical), it is certainly much easier to stop 6" from the top and finish with dirt. And, as a bonus, you won't have to go down so deep, 32-36", for common posts, works just fine here.
As I said, this is proven science here. I have sent a lot of time fixing heaved posts, installed by others. None of which were a problem afterwords. Your welcome however, from here on out, to encourage those who supply me with work, by suggesting your method.