Why do some editors/journals withhold response letters from reviewers?
You learnt that the  authors' response was essentially "we accept the reviewers comments and have revised the manuscript accordingly."  If such revisions were clear to the editor there's no need for further review or communication.  

Don't forget that while you may turn reviews round quickly (or you may not), there are plenty of reviewers who even for a second review  have a high chance of sitting on the paper for an unreasonably long time.  This (on top of workload) is a further incentive for the editor to minimise unnecessary links in the chain.