I think this proves quite the opposite. As a developer, you can decide if you want to change your users' default search engine (which will probably annoy them a lot), or do anything you want. It's a platform.
The fact that Crossrider provides APIs for doing things like that is okay.
My general (perhaps theoretical) concern is that Crossrider is putting themselves in a powerful position by having their software running on 100 million peoples' machines (likely less since some people have installed multiple Crossrider extensions). If Crossrider decided to go evil and start monetizing those extensions, as an extension author I would likely get the blame since the user installed my software and not Crossrider.
A lot of this likely depends on what their business model is. For example, is Crossrider spying on users' browsing experience and monetizing that data? If not, do they have the right to do that in the future?
At least that's my understanding from reading about Crossrider on Reddit (http://www.reddit.com/r/technology/comments/inkl0/crossrider...) where they alleged, for example, that Crossrider extensions changed users' default search engine.