If shields are damaged when actively used to block...
... why aren't weapons damaged when they strike?
... or armor damaged when hit??
I love the idea of Raise Shield / Shield Block making shields more interactive than before, but the disparity of taking damage when weapons and armor do not doesn't make much sense.
It doesn't really bug me, but if you want some sort of reasoning, picture the crumple zones in a car. Those parts of the car are not only not designed to not take damage, but they're precisely designed to be the things that take damage in the event of a crash, to absorb much of the kinetic energy. For a weapon, you want to pass on that kinetic energy onto whatever poor sap is on the other end of that weapon, and Armor is so close to the wearer's body, that deflecting that energy makes more sense than absorbing it.
Is this a bit anachronistic? Sure, but let it never be said that Pathfinder attempts to be a historic representation. In fact, I'd go so far as to say, in the lore of Golarion, it makes perfect sense that some intrepid blacksmith or what not discovered this, in universe, due to the relative length of the period of pseudo-medieval warfare, as guns, which were a main factor in the abandonment of the shield, are really uncommon in the larger landscape.
I actually credit the way Shields work as the reason I screwed up and made armor provide and AC bonus and hardness as DR. I read the rules for damaging items, didn't realize it was exclusive to Shields and was like, well if they are taking damage regardless my players can have reduced damage to compensate.
Good news! That line in item damage was removed in Errata Update 1.2!