@marcg I agree manipulation of any dns should be opt in for sure.. If I want you to filter stuff, or help with my typo's etc.. I would point to the IP specifically that you offer those services up on.
If you offer such service, I should be able to point to any other dns I wan't and your not going to mess with the traffic - shouldn't have to opt-out if not pointing to your dns that is for damn sure.
Default opt-in is rarely a good thing for anything.. No matter what services they are providing - the user should have to opt-in in some way if you ask me.
One of the pet peeves I have with doh - many of these browsers like to turn it on by default, which is not the way to go about it.. If its so good inform the users and they will enable it if they want too. If you turn it on by default - tells me its not so good of a thing.. Seems like to me your trying to sneak it in under the radar.