The extension did more than just add ads. A javascript listener for the click event was attach to each link on the result page.
If the user hovered with his mouse over a link, he couldn't tell the link would lead him to http://www.ecosia.org/. But this is exactly what was happening, because the click listener was changing the URL only after the user clicked.
So now the user was redirected to http://www.ecosia.org/ along with a bunch of parameters, including the original query and the original URL, and from there http://www.ecosia.org/ redirected the user to the original URL (after logging whatever it wanted to log), without the user having a way to notice what had just happened (unless looking in the dev console).
The fact that the URL was changed only after the user clicked is quite a hint that deception was intended there.
Paul was talking about a different extension, but anyway...
The onclick event listener is the same thing Google does with the search results. Perform a search on Google and right-click a link, then you'll see the URL changes to the a Google proxy server that collects data about your click for analytics purposes. The reason is so the whole process is more transparent and the users can see the actual URL they end up with when clicking the link. The intention was not to hide anything, but to keep things as unobtrusive as possible. I'm sorry if it felt any other way!
If the user hovered with his mouse over a link, he couldn't tell the link would lead him to http://www.ecosia.org/. But this is exactly what was happening, because the click listener was changing the URL only after the user clicked.
So now the user was redirected to http://www.ecosia.org/ along with a bunch of parameters, including the original query and the original URL, and from there http://www.ecosia.org/ redirected the user to the original URL (after logging whatever it wanted to log), without the user having a way to notice what had just happened (unless looking in the dev console).
The fact that the URL was changed only after the user clicked is quite a hint that deception was intended there.