For what reasons (if any) would a journal refuse to inform the reviewers about the final accept/reject decision?
One possible rationale would be a belief that the acceptance decision ought to be a confidential matter between the journal and the author, and the reviewers simply don't have a &quot;need to know&quot;.
The argument &quot;it is important feedback for the reviewer&quot; isn't very strong, in my view.  Knowing the fate of the paper doesn't really give you much information about whether your review was helpful in reaching that decision, and certainly doesn't tell how you could have made it better.  Conversely, if the editor wants to give you feedback on your review, they can certainly do so without revealing the final decision.
By analogy, consider what happens when you write recommendation letters for people.  When they apply to a school or employer, the school doesn't tell you whether the applicant was accepted or not, because they consider it to be nobody's business except the applicants's (and privacy laws may back this up).  Of course this is a different situation in many ways, but I think the underlying principle is the same - having been part of a process doesn't, in and of itself, entitle you to information about the result which would otherwise be confidential.