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.
If you're smart enough to follow a few simple instructions and install a local extension, you're smart enough to follow a few simple instructions and install the Dev channel of Chrome first.