It happens more frequently that people react like this if something they believe in, turns out to be fake.
There has been a case of a “doctor” who told patients with a tumor to eat raw grass and play the piano. His personality was such that people really trusted him. When those patients were told that this treatment had no effect at all, they became quite angry.
So, back to the subject.
Oxygen in itself does not cause cancer. True. But hydrogen peroxide is not oxygen. Hydrogen peroxide produces oxygen. And water. The chemical reaction of the decomposition of hydrogen peroxide into water and oxygen is:
2H2O2 –> 2H2O + O2
In plain English: two molecules of hydrogen peroxide decompose into two molecules of water and one molecule of oxygen.
Now this reaction equation only shows the starting product (hydrogen peroxide) and the resulting end products (water and oxyegn). The problem is what happens during the decomposition process. Halfway through this process, free radicals are formed. Those are the thingies that are the most powerful oxidants, and the most powerful cell-damaging chemicals, we know of. Those free radicals are the very reason why the body so desperately needs antioxidants, like vitamin C and many others.
There really is no safe dilution of hydrogen peroxide. Just like with any other carcinogen, if you ingest low enough quantities, you will not directly notice the carcinogenicity – but it is still there, albeit lower than if you would have ingested a stronger solution.
Your example of hydrogen peroxide being added to the water lines has nothing to do with feeding livestock with hydrogen peroxide. I do not know about water purification methods in the USA – I think you are quite fond of chlorine – but I can tell you that in Europe, one of the methods that is routinely used for water purification is injecting hydrogen peroxide. Or ozone, which has the same effect: ozone too decomposes into oxygen, with free radicals as an intermediate step. Just like hydrogen peroxide.
Anyway, the very reason for injecting hydrogen peroxide in the drinking water of your livestock, or in European water purification installations, is to kill pathogens. The hydrogen peroxide decomposes into water (no problem) and oxygen (no problem), meanwhile producing free radicals, and those free radicals kill every living micro-organism in the water.
The hydrogen peroxide is, of course, long gone before the lifestock (USA) or the population (Europe) drinks the water.
The oxygen, by the way, also evaporates from the water before lifestock or people drink it. Thus, the one and only effect of adding hydrogen peroxide to water is to kill micro-organisms. Just like when adding chlorine, but hydrogen peroxide has the advantage that nothing is left – very unlike chlorine.