That's just creepy. Replacing page content, unless ad/spam blocking or specifically user-selected filtering, is completely unethical and totally falls under malware in my book.
Good on you for turning that down. You've saved your users a lot of aggravation.
If you think about it: as formulated, virtually all present Web advertising works via content injection. You're just (nominally) limiting who gets to insert content.
Good on you for turning that down. You've saved your users a lot of aggravation.