Clean Reader does a simple real-time search/replace operation on ebooks that you own. Some writers have argued that an app like this violates an author's moral rights.
The doctrine of Moral Rights varies from territory to territory, but it's a heck of a stretch to extend it to this activity. It's one thing for a publisher or retailer to send out copies of your books in which words are changed around without your permission. It's another thing altogether for the reader themself to decide to read their legally acquired books in such a way as to change the text.
Imagine a website ("ebooktriggerwarnings.com") that indexed all the pages you should skip if you have experienced trauma and want to ensure that you don't read rape scenes. There's no coherent doctrine of moral rights that would prohibit readers from discussing, indexing, and sharing this sort of annotation, even if it leads the readers to miss out on whole passages when they read the book.
It's a truism of free expression that if you only defend speech you agree with, you don't believe in free expression. That doesn't mean you have to defend the content of the expression: it means you have to support the right of people to say stupid, awful things. You can and should criticize the stupid, awful things. It's the distinction between the right to express a stupid idea, and the stupidity of the idea itself.
I think Clean Reader is stupid. I think parents who want to ensure that their kids don't see profanity have fucked up priorities.
I think readers should be allowed to skip my foreword and author bio. I think they should be able to search out their favorite passages and read them out of order.
I think racist readers should be allowed to make an index of "scenes that racists find disturbing," so that other racists can avoid them. I think those racists are fools and worse for doing it, and I will condemn them if they do. I just won't say they're not allowed to do it. A rule that says this kind of list is prohibited would also prohibit a the same list, compiled by anti-racist activists, under the heading, "Scenes with which to annoy racists."
I think readers should be allowed to annotate my books. I would love an ebook reader that lets climate scientists rebut the claims in a climate-denial bestseller, and lets readers decide whether or not to see those annotations when they read their own copies. I support this, even though it would allow climate deniers to mark up books about climate change in similar fashion.
You have the right to rearrange the words on your screen in private. This goes without saying. You have the right to insert serial commas in my sentences. To change "Ios" to "iOS" (or vice versa). To line out the profanity and replace it with stupid euphemisms. I might call you a fool for doing it, but I would never say you should be prohibited from doing it.
I believe in this without limitation. Another writer raised the spectre of having my words changed to advertisements. If the reader wants to put ads in a book she has legitimately acquired, it would be weird and inexplicable and foolish: but it should be permitted.
After all, if you think readers should be prohibited from inserting ads if they choose (though again, why would they choose this?!), then why shouldn't they be prohibited from blocking ads if they choose?
(Image: Redacted, Opensource.com, CC-BY-SA)