I agree it wouldn't be a problem really for them to xxxxxx out stuff they don't want you to see. At least it is evident what happened. I don't like the idea that you can (must, in fact) buy books through the app, and those books get
modified -- it seems to imply that such changes are in fact the will of the author.
Likewise, I accept that anyone can delete, redact, or
modify any damn words they like, themselves, on a DeDRMed EPUB. They will know exactly where those modifications come from, and it isn't the author.
Also, you could always do that with pbooks.
If this app were just a reader app, that allowed one to sideload books and have them cleaned (for a given value of clean), again I wouldn't really have a problem with it. It is no different from doing it yourself, except that you enlisted someone else's help.
That being said, it may not be illegal (false representation of the author), but I will, regardless, always think it immoral and mentally unsound... but that is just an opinion, after all.
Having this app as an integrated vendor experience seems rather shaky, legally speaking -- although I am no expert. I certainly hope so, anyway.