Strange. The law seems to imply that the Austrians don’t trust themselves. If Hitler had offspring, would it behoove Germany to ban them from office? Would Americans be afraid of a direct descendent of George III (born on American soil) running for POTUS?
Maybe wait a couple of generations. I’d be more worried about cronies of some bad king wanting to put the king’s son on the throne so they can continue their corrupt business. Like if Saddam Hussein’s sons had been allowed to run for office in the new government of Iran. (I don’t know if Saddam’s relatives were actually barred or not.) But if it were Saddam’s great-grandchild, who cares? The cronies of Saddam would not be there any more and the great-grandchild would probably have to win on his own merits for his own reasons.