This has never been about security. When two-class domestic 757s are sold as single-class service on routes such as BOS-MAN, anyone can use any lav. (This has been documented via AA internal materials linked to from earlier FT posts and is borne out by my personal experience on that route.) If having someone from Row 25 use the lav behind the cockpit was a security risk, it would still be one if the folks in the front cabin paid economy class fares.
I applaud anything that reduces the number of times people say "security" to justify something that has nothing to do with security. So often it's just a way to shut people up and avoid having to answer legitimate questions.