I think exploit is not really a accurate term.
Its more that its probably being used as intended but in bad faith; I pondered why SE allowed this to work - reality is if SE wanted to stop this they could very easily - the flag they set for personal housing is a character file flag that associates your character to your account (think of your account name as your character and every "WOL" you make is just a alias of your account) the flag is world specific - this is why you can't have more then 1 personal per world. SE decided not to apply this same limit to FC's which is a odd choice but I suppose it does allow friends to FC sit for their friends while they submerge their FC's until the last content delay concludes. This is also contingent on those FC house-sitters logging in and visiting each of those houses roughly every month (which is easy because if they are running a sub-farm then they are already doing such.)
As such its likely not a exploit - just not in the spirit of the mechanic; SE could easily stop it from happening; its not like they cannot see multiple wards in game that are all ran by the same 4 accounts. If SE isn't doing something that's a choice; not because SE is helpless and if it were a violation of their rules they would have acted by now; SE has quickly responded to far less significant rules violations in the past and this one would probably be a pretty easy fix with a simple patch to have FC houses treated like personal homes by setting the same flag. Note that it's likely not a exploit because even if someone has a FC on that character; they cannot move that FC without first starting over.
Personally I think a easier fix would just to allow people to consolidate all workshops in 1 location and then apply the aforementioned flag.
Also note whats good for the goose is good for the gander; so if we applied those rules it would also apply to the Kingdom of Noctis (I think thats what its called) On Spriggan; which is actually a beautiful accomplishment where they turned a entire goblet ward into a full blown city with amenities and made the Goblet look much nicer.