This allowed companies like WPEngine to exist... and focus their resources like marketing and maintaining their internal "fork" rather than contributing to core Open Source Project... and this might be one of the reasons they are getting some good success.
Generally trademark is what should offer some protection here and I think this is where Open Source landscape was not tested well.
WP Engine claims on their front page they are "Most Trusted Wordpress Hosting" which arguably makes folks to assume they host full featured Open Source Wordpress, which per Matt's article does not seem to be the case.