I dont know the answer but I can see where you are coming from.
It seems so stupid that you cant stop just for fuel given that you are not leaving the airport confines.
Looking at it logically, I can however see a valid argument against.
There are plently of grass strips with fuel where you could stop just for fuel. However, these are not sterile areas.
You could claim you were stopping for fuel whilst dropping off half a dozen illegal immigrants who would be through the non existant perimeter fence before they could say Ooo la la.
In theory this would not be possible at larger customs airports becasue airside is sterile and at smaller customs airports, customs might be watching you!
Of course the stupidity of the whole process is highlighted by several of my trips to France. I now routinely ask the Customs airport whether they want me to stop, or whether a T and G and or low level fly past will do. Often they chuckle, tell me customs are not here today anyway and be on your way! Is that legal - I have no idea, but if ever I am asked I shall say I was told by the Tower not to stop. (not much help if you only want fuel I agree).
Compare the hoops we have jump through with yachties who I suspect could cram far more immigrants into their focsle that we could fit in a spam can.