If you attempt to install any extensions in Safari 11 and the only source is the MacOS App Store. It appears Apple is vetting Safari extensions for Mac the same way they do on the iOS App Store so they can enforce their security and reliability rules.
I've been wondering about that for a while... iOS was a newly created platform, and I suppose Apple had the right to impose any restrictions on it that they wanted, but "freestanding" extensions have been permissible for years, and any attempt by Apple to totally restrict them for any reason could be considered censorship, perhaps even monopolistic. I certainly hope they'll continue to allow user discretion as they have in the past.
