Google is locking down Chrome for Windows users by blocking the downloading and installation of any Chrome extensions not found in the company's own Chrome Web Store. Google says that malicious extensions that take advantage of the option to side-load additional extensions upon install are the "leading cause of complaints from our Windows users."
Chrome engineering director, Erik Kay, posted on the Chromium blog, "Many services bundle useful companion extensions, which causes Chrome to ask whether you want to install them (or not). However, bad actors have abused this mechanism, bypassing the prompt to silently install malicious extensions that override browser settings and alter the user experience in undesired ways, such as replacing the New Tab Page without approval."
The lockdown will go into effect in January 2014.
Source: Chromium Blog