Technologies like MediaMask and ClearPlay don’t actually alter anything. They use a script, developed specially for each movie, that tells a compatible DVD player (or their own DVD player software) which parts to skip or blur out. These scripts are provided by the manufacturer as new movies come out.
For comparison, look at the program feature built into most DVD players. You can tell your DVD player “play chapters 1-10, skip chapter 11, and then play chapters 12-20”. If you were an overprotective parent, you could find a list of which chapters on some new DVD contain violence or nudity, and only let your kids watch it if they skip those chapters. These technologies simply do the same thing on a finer scale - instead of skipping an entire chapter, you can skip a few frames, or block part of a frame from appearing on screen, and you don’t have to program the list by hand. The actual content on the DVD is the same; you’re merely changing how your equipment interprets it.
However, that defense doesn’t apply to those who are actually selling sanitized tapes and DVDs.