The quickest way to get a child to stop swearing is to tell them what the words actually mean. Once they know what the words mean, they cease to be bad words and become offensive words. Since very few kids intend to be offensive when swearing, they simple stop using those words. (Yes, I have done this. It works almost every time.)

Now parents don't want to expose their kids to these words for various reasons. Some or those reasons are trivial things that they should be able to deal with, while others are not. Either way though, those books are exposing kids to these words and they are doing so without context. They don't mention what the words mean and when it is inappropriate to use them. Take that darned word damn. For a lot of people it doesn't have a deep meaning, either because it doesn't reflect their belief system or because they realize that the user is just expressing themselves emotionally. Yet if they use that word in a particular manner on a Christian person with a more uptight disposition, then they are bound to elicit a very negative response. The child does not understand that because they rarely know what the word means to certain people. The same can be said for practically any swear word. As such, I can understand why a lot of parents want to shelter their kids from those words.

As for whether this is censorship or not. I don't see what the problem is if a person is using this software for their own gratification or because they believe it will protect their own child. In the latter case it is because the parent is responsible for the upbringing of their child, and are often left dealing with the consequences. Used in a more general context, to censor books for other adults or for other people's children, then yes it is a problem. As others have pointed out, this is probably an issue from the perspective of authors as well -- since changing the words can change the meaning.