Not in my opinion. I think you owe the user a clear indication about that it is an add, and from who the add comes. Just modifying pages like that is malware behavior.

If they clearly tell me it's their add then I can chose if I want the extension or not, so removable adds is less of an issue.

Yup. I would still have happily used it. Now there is just no telling what else they may do without telling users.