This may be beside the point, but there is another issue I have noticed that may add to this argument: the ability to promote yourself (or demote those above you) if you have promotion allowance. I have heard of many fc's having an issue when one of their officers decided to go rogue and invite/promote hostile people into the fc for the sole purpose of stealing. Having the ability of promotion is essentially having the ability to change all rules, since (unless this has changed, I haven't checked in a while) you can promote yourself. From what I understand, this did not occur in this instance, but it has been a problem for others, and I don't understand why such a system made sense to the devs in the first place...
That all being said, fc's should be mindful of every allowance a specific rank has, and should be aware that in a game environment it is the mechanics of the game itself that protect you (or leave you wide open), not agreements among players. Most of our friends in MMO's are in truth strangers in reality, and the rules of these small worlds are vastly different than the real world. Do I think the GM's should be better equipped to deal with problems such as this? Absolutely, having such a glaring hole (this has happened to many fc's) is a huge problem, and OP is correct in the fact that other MMO's handle such things far better, sadly. But this does not change the fact that the perpetrator was allowed to do the deed by the other officers due to negligence of the current terrain and minutia of the fc mechanics, poor though they may be.
I agree that SE needs to figure out a way to keep things like this from happening, but the blame does not fall with the GM's or their ability to "punish". My apologies to those who were affected by this theft, it really does suck what this person did to you, but the response by the GMs is adequate. Who you really should be going after is the devs for having a very odd system in play for fc rankings...but even more-so, because of the limited system, you should really only have 2-3 people in charge of most allowances in your fc, and prepare for the possible loss of anything that goes into the chest just through daily play, let alone by anyone with vile intent.