What's changing the text is the app - and it's that which may be suspect under copyright infringement as the author has not authorised the 'translation'.
BUT - they're not free to alter them for others - that's a fundamental tenant of copyright - they're not free to make a changed version of the text for profit or otherwise) which is what this app is doing and why its so scary.
What do you think of Google translate or those apps being developed that can translate things on the fly from one language to another? One could say that those things infringe on language/translation rights, limiting an author's ability to sell their stories to foreign markets.
No doubt the app is 'safe' because it's only an enabling bit of software that sneaks through the cracks - but it goes against my main beliefs that things should not be white-washed - and that we should be adult enough to see things for what they really are. And then either accept them or reject them.
I dislike it probably as much as you do. Hearing/reading a bit profanity IMHO isn't harmful. But to me this program falls in the ballpark of "I do not agree with what you have to say, but I'll defend to the death your right to say it."