Wow. What a massive overreaction of a thread.
When you visit a website that's not in that list, and click on a Unite app download, Opera gives you a great big warning, and insists that you add that site to the exceptions list before it allows you to download the file from the previously un-trusted site.
When you visit one of the sites listed there (all on *.opera.com) it DOES NOT give the site the ability to install anything silently behind your back or any such nonsense - it simply means that when you click the install button, you get an actual install dialogue and not a big fat scary warning blocking the install dialogue.
For anyone familiar with Firefox, it has exactly the same system for its extensions - ever tried installing an extension from somewhere other than addons.mozilla.org?