I suspect that this is less about blocking YouTube downloaders, and more about blocking those extensions that appear after not un-checking the box on programs downloaded and installed from the internet.
I.e. it's more for the protection of grandma who wants to download a pretty solitaire app than it is for stopping little Johnny downloading his music videos of Miley.
This. Google is not stupid, they know that users that want to install extensions from their disk know how to install Chromium. I'm fine with it if less crap is going to get installed on my parents computer. I'll just install Chromium and everything will stay the same for me.