Interesting. So my questions would be:
1) Is deleting/severely altering your initial post against the rules?
2) If not, does it actually need to be, to merit action against someone by the mods?
3) Whether it is or isn't against the rules, should the rest of us develop the habit of quoting the OP's initial post in full, to lock in the original text?
My perception is that the mods here have a pretty wide latitude when it comes to chiding/suspending/banning someone. We're not in a situation where people are going to parse the rules and say, "No, you can't suspend me for that, there's a technicality that means I wasn't doing anything wrong!" and have that be an effective argument.
If someone blanks their first post as part of a pattern of protesting the advice they were given, so it seems like they're flouncing off, that's just part of their larger attitude problem, which rightfully should be addressed. I don't see a lot of people blanking initial posts because they truly think that's the right thing to do; and when it occasionally happens (only once that I recall), a clarification sets them straight that this isn't how we do it. If someone starts to argue about it in a disrespectful way, that in itself becomes the problem, because people here need to handle disagreements politely.
All that to say that, even if it technically isn't in the rules, many of us feel the problems with blanking your initial post are obvious, and usually are addressed by a mod when they're alerted to it happening. Just because the mods don't address something publicly, doesn't mean they haven't addressed it privately with the poster, or otherwise made an active decision about it. In other words, I don't see the lack of public censure on recent threads as being the mods condoning this behavior, I guess.
As for point 3--I don't see anything wrong with individual posters deciding they're going to do this, provided they use good judgment. If the initial post has already been quoted in full earlier in the thread, no need to do it again, especially if it's extremely long. It's mildly annoying to scroll through long quotes before getting to the new content (IMO), and I have a computer with a larger screen, so I can only imagine how much more difficult it is for posters using a cell phone or something else small. I don't feel like it needs to become a rule itself, though, like in the sense that if I'm poster #2 and I forget to quote the OP, poster #3 is justified in chiding me for that. Poster #3 can just quote the OP themselves, and it's done.