They have become used to it. The whistling, booing, sneering and yelling is predictable. Still it is not a very welcoming reception, of course. And remember: Letizia is from Oviedo! The city is also capital of the principality of Asturias, which the King was for decades! If even in their very own Oviedo they can not be received without all this disruption.... Yelling to a daughter of the city, to a fellow-citizen from Oviedo.... Other royals would also show a stiff upper-lip but possibly feel shocked. ("What have we done to deserve such hostility?")