The question is what would be a reason to scramble posts dates during copy and paste, not what is the reason, just what might be a possible reason. Not everyone understands what a hypothetical scenario is, but purely from a theoretical basis to answer the q it might be worth considering a case in which fb wasn't goldilocks. So, I am not saying any of below is true, just like considering like x=ai+b (imaginary solutions) to a question of what is the root of a quadratic equation where the solution is really known to be real. If that is confusing please stop reading now.
That out of the way, purely hypothetically speaking, what if
(1) FB wanted a semi-monopoly on downloading user's own data, so that you'd need to make a request to them and their software, or a human, could have a chance to edit it as they wished before hand? Hypothetically.
(2) FB wanted to alter dates without people knowing?
(3) FB wanted to de-index some posts they didn't like?
(4) FB feared article theft, where search engine optimization "authors" would use a scraper to copy and paste high quality (some posts can be long on fb and one could use likes and size to get an idea of quality) content from facebook?
(5) FB wanted to know who was copying what?
It's purely hypothetical, but let's say a bad event occurred, and the exact time of some post mattered. You know, like a court case where they ask these sorts of questions. Would #2 give fb more power? Hypothetically? What if the power is just to know that one requested a download of their data, as in having a list of all the users who don't really trust fb to store their data?
Is it really true that the view of someone logged in is the same as the view of someone not logged in, or are the number of visible posts drastically limited for those viewing anonymously?
What features would be helpful in the hypothetical goals #1-#5?
One feature for both might be a non-direct relation between what the user gets when copying text manually (after expanding posts, itself a potentially related "feature") and what the computer saves into say a word document upon a paste command. It would make it more difficult for people from just doing that simple method, and boost the role of making fb be the middle man. There are historical cases where an entity wishes to be the middle man, so it is not as insane as it seems. An exact example is the app called com.mhd.flasher.n54 which allows one to modify some engine parameters. (You have to go online and enter your vin number. You cannot just hack your car without going through the middle man.)
Is that the reason? No idea, but I did think the other answer is not the only possible answer so I wanted to post this one.
What if the other characters might be unique to the copy so that the time of the copy and who did the copy can be known. This is super easy to test of course but I'm lazy and just wanted to post other more creative answers than the "this is a dumb question" (the supreme brilliance of something as shocking as occam's razor, just a glitch) answer sitting here before us now.