If it was the free market at work, the app would have failed in the marketplace because people refused to use it. This situation quite different. The revenue generating component of the app was disabled because a vocal group of people decided that they did not like what they saw.

I don't see much of difference between that and removing books from libraries. In both cases you have a group of self-righteous people who claim that they know what's good for everyone trying to restrict access to a product. In both cases those people are attempting to ensure that other people do not have the right to make their own decision.

The right to make their own decision, are the key words here. In this case, the users of Clean Reader had to make all of the decisions: they decided to obtain this software, they decided to use it, and (judging from the screenshots on Google Play) they determined the amount of filtering while reading.

If you don't like it, fine. You don't have to use it. I don't use it because I don't think it's a terribly good idea. Yet that does not give me the right to prevent other people from using it. I wouldn't want to prevent other people from using it anyhow, because I understand that there are uses for the software.