Why complain? well because that is what people do. Not matter what Bethesda adds/removes there will be some people complaining. Bethesda implemented the respawning due to the complaints that in Oblivion there was anything (i.e.dungeon crawling) to do after most of the quests were done, now people are complaing about the respawning. You see some people in here complaing that the game is too easy and some complaining it is too hard. As long as there is the internet and these forums people will complain.
Don't get me wrong, this is no reflection of what I really think about this game (it is actually magnificent in it's near entirety).
You're right though, in Oblivion we complained because nothing respawned. And now we're complaining because EVERYTHING respawns (hell, even picked locks respawn for crying out loud).
What, did the undead reset all the traps, lock all the doors, and raise all the bridges after their bodies magically reanimated...then crawl back into their crypts? Am I being ridiculous in my observation? No, I'm not. Am I demanding perfection? Not at all. Bethesda is doing great in listening to the community, but this is just one example of the "opposite extremes" they go to in order to "fix" things. I know they have a lot to juggle with a game this big, but put a nanosecond more worth of thought and you'll hit the nail on the head with this kind of stuff.