Mutt - that wouldn't surprise me in Doha ! I'd agree that placarding a fuel tank is not a matter to be undertaken lightly actually.
I hear the sense in the possibility of confusion with diesel/A1 powered SEP but I'm guessing such aircraft are well placarded for that very reason and have little need of this rule. On the other hand a Citabria owner might have very little desire to have some crappy label done in powerpoint with god knows what adhesive on being stuck to his fabric.
Seems to me though that airport management aren't the right people to be the placard police and its somewhere between arse covering and ineffectiveness. If they really cared a refresher/toolbox talk for the fuelers would be the way to ensure attention. If it really is about safety rather than back watching , they could ask pilots to explicitly tell them what grade they wanted or sign the fuel chit before fueling.
And yes, why would we be more trusted to buy the right decal than check the fuel itself?