I know there are a few apps which will check the clipboard in order to provide functionality to the user. For example, some shipping apps will check the clipboard to see if the user has a copied tracking code and if so, ask the user if they want to track their copied code. Not sure if TikTok does something similar, but there are certainly innocent reasons for checking the clipboard.
I'm disappointed that so many people think "hey they could just be doing this for innocuous reasons" instead of "oh maybe nobody should be doing this even if it's the absolutely most straightforward way to do it." Even if you're only looking for a shipping tracking number and then only so that you can provide useful auto-populate, will you lose out by only checking the clipboard when the user hits your text input fie…
It wasn't offensive to the user before iOS changed the rules. It is just a technical detail behind a small feature.
Every app can do bad things. For example, every app with a password field can use that data to crack your account on others services. You shouldn't reuse passwords but we all know that too few people follow that rule.
If you installed an app from some company, it means that you trust it to some extent and with that in mind it is reasonable to think that the issue is innocious. If you think a company wants to steal your passwords, why did you install its app in the first place? Clipboard or not, it will find a way of doing bad things.