Last year, Mozilla started fighting back against third-party add-ons installing themselves in Firefox without the user even knowing about it in many cases. Firefox now checks for offline installs and disables any add-on the user didn't specifically choose.Google has now started working on the same thing, albeit Chrome doesn't handle third-party extensions as well as Firefox does just yet.
On the one hand, Google blocks any extension that doesn't come from the Web Store or is at least available there. Chrome extensions from third-party locations can't be installed, at least not without quite a lot of hassle.
The Chrome team has added a new feature that prevents sideloading, i.e. installing extensions from outside of Chrome.
The latest Chromium builds now notify users that an extension has been added by some other app.
The extension is also disabled by default, unless users decide they want it on. The feature should be landing in Chrome 24 dev soon, but it's going to be a couple of months until it makes it to the stable channel.