There are two differences. The first is that you have to replace "preconceived notions of how the universe works" with "vast body of experimental data and scientific understanding of how the universe works". Our evidence for how physics works vastly outweighs current parapsychology research results. The second difference is that the evidence is not being ignored (at least by the author of the article), but is being taken to indicate a real effect, it just isn't the effect of psychic powers. Our knowledge of physics means that the existence of psychic powers is given a much lower prior probability than the possibility of widespread experimental error and bias. Since both explanations are likely to produce slight positive results, the existence of psychic powers is still unlikely once you take the evidence into account.

However, the important point in the article is that in order to make this inference in an intellectually honest way, you need to significantly increase your estimation for the probabilities of widespread experimental error in ALL scientific studies that use similar methods. Since the methods in parapsychology are pretty good, this has quite a far reaching effect.

In a sense, your question about "on what grounds can people claim one field to be nonsense but not others?" is exactly the same question asked by the author. Except the author isn't implying that parapsychology isn't nonsense, they are implying that many other fields are nonsense too. This makes sense because physics is almost certainly not nonsense.