Stefan, there's a good reason for this policy. Feedback is double-blind, meaning, neither the freelancer nor the client know what feedback was given until both do so. But if either client or freelancer could subsequently change feedback without asking the other for permission, then a freelancer who received bad feedback could then retaliate by lowering the feedback she or he originally gave to the client, and vice versa.
"No good deed goes unpunished." -- Clare Boothe Luce