I think a lot of this comes down to beliefs on what a review is, and what the purpose is.
To me, a review is meant to be a tool that a consumer can use to decide whether or not to buy something. It is meant to do this by having somebody attempt to be unbias, and giving their critical evaluation of the work. This does not logically exclude the possibility that a writer may be able to view their work objectively, but, at the same time, the fact that the writer is so attached to their work means it is very unlikely. Thus, from a consumer’s point of view, a review from a writer is not very helpful, as the writer is more likely to be biased. Some people use reviews to guide them when they want to buy something, and they do that because they take them to be “what other people like me have said”.
I think that a writer should be able to talk abuut their own work, but the term “review” has an implication of objectivity/distance from the product. Perhaps a “critical reflection”, would work better for when a writer talks about their own work, or something else entirely. Giving a rating in a system where that will influence the overall rating seems slightly wrong as well. I always took those kinds fo star-ratings to be “other consumers have said…” If that result is then altered by people who have worked on the product, or have some investment in it, then it gives a less acurate reflection of what other readers think of it.
Psychologically, I think we feel safer taking advise from other consumers. Even if it’s not the writers intention, I think some people can feel “dirtied” from the idea that somebody is trying to take advantage of the trust put in reviews.