Exactly. Those bundled offers are by design deceptive. When I buy a car I don't expect to be receiving a gym membership because it's written in the small prints of my purchasing contract.
That's what I did in the end since there was no other way. Seems to have done the job, I can't see anymore traces of Avast, even in the registry.
Still, I don't see a box to opt out in the customize menu of CCleaner. If you don't opt out on the first window, you're screwed.