Normal apps wait until the user attempts to perform a paste action to access the clipboard, instead of accessing the clipboard every two seconds.

TikTok reading it every couple seconds is definitely excessive. And frankly our tools fail us by not at least revealing that it's taking place -- new clipboard notification aside, as shown in TFA. How do I know how common vs weird this behavior actually is?

I'd only refine your post to say that it's common for apps to read the clipboard without you pasting. Right click Chrome's omnibar and it will show "Paste and go to <clipboard contents>", my bittorrent client and RSS clients prepopulate the new torrent/feed form if I have a URL in my clipboard.

Is the tiny convenience worth the ability to snoop? I don't think so. Or rather, I would like to decide that for myself.

Malice or just stupidity? I can imagine a dozen different reasons a program might access the clipboard in a loop, all of which reduce to "we are bad programmers".

Not all of them reduce to bad programmers. It could be either of malice or stupidity. In this age when data is valuable, it is better to be safe and assume malice.

It is the OS that should transfer the contents of the clipboard to an app, when the user tells it to do so, and not the other way around. The content of my clipboard shouldn't be any app's business until I decide to paste.

To be fair, I don't think the programmers implementing system APIs expected them to be (ab)used like this.

Now system API developers have to view downstream app developers in an adversarial manner...

Ok, but your beef is with Apple, not TikTok.

My beef is with the person who installs a spy camera in my shower, not with the person who didn't expect someone to install a spy camera in my shower. Only one of those people actually installed a spy camera in my shower.

Yeah it seems like it. Chrome and a lot of other apps show an insane amount of these popups on IOS14 to either actually check your clipboard or just to toggle "paste" buttons.