this is nothing new, these apps have been out there for a while now
If you're referring simply to the screen shot capability, say on an iPhone, then a Snapchat user is informed when you take a screencap using that method. I think that once upon a time the user was not notified but Snapchat must have updated; I confess I am an avid user of the medium and I am notified when my friends take screencaps using the typical methods.
I honestly don't really know why it matters if the person knows you took a screenshot or not. You're still taking a screenshot of something that was (probably) not intended to be screenshot. And if you're using Snapchat you should be aware that other people CAN easily screenshot what you're sending and hopefully trust that those you send pictures to won't.