It used to be that taking a screenshot briefly stopped the touch event, so it could notify the sender when you took a screenshot.

iOS 7 fixed that bug, but added an official way for apps to detect screenshots. I would guess snapchat has just hung on to their old interface out of tradition, since it still works. It just doesn't have anything to do with screenshots anymore.

Ah, that makes sense, thank you.

The point of it is to reassure unsophisticated users.

It also attempts to notify the sender if the receiver takes a screenshot of the message.