1) you cite two human studies and a rodent study. Neither human study has a placebo group. This is generally considered unacceptable in studies of pain control because both "active " medications may have placebo effects that can be considerable in subjective scale studies like these. We can't tell from these studies whether a placebo would have caused the same degree of symptom relief.
2) both studies are sponsored by drug companies; in fact, in one of them the authors thank the drug company rep for design and data analysis of the study!! What you need to know about drug companies is that they reserve the right to publish or not publish a study they pay for. So they may have commissioned 20 identical studies done by different groups, with 19 of them being unpublished and showing no benefit and this published one showing possible benefit.
3) in the rutin study the authors point out that the enzyme is quickly metabolized to homovanillic acid (a dopamine metabolite) and other products. They profess they have no idea why an enzyme would work. Why are such studies done, you asked? Because the companies that make the product pay people to do them.
4) the rodent study is irrelevant as rodents have very different GI tracts than we, and may for example absorb some peptides directly from the roof of the mouth, for example.