Originally Posted by The Pook
Chromium Engine is open-source, Opera (or any other Chromium based browser) can just choose not to implement it.
Hahahaha! Good one!
Wait... you're not joking...
Google owns the Chromium project. The changes are made on the Chrome API and if you don't implement them there are no more extensions for you adblocking or not. It being open source only means that you can fork it if you don't like the way it is going but I guess that Opera and others chose Chromium as their foundation because they can afford to develop their own.
Just to put things under perspective, Firefox is open source too and when they changed the add-on system everyone had to adapt to that or fork and get out the door.