to be honest, I think the main reason is that they probably dont want any gangs etc hanging around the graveyard. You will be surprised when a bench is suddenly put somewhere how many people hang around for all the wrong reasons.
Therefore I agree there shouldnt be one. At my loved ones grave, if I want to see and talk to them, I usually bring a little blanket and sit at the side of the grave..
Its terrible in this day and age that you cant have such a bench, but sure look what happened all along the liffey, that lovely walkboard ruined with junkies hanging around the benchies...