I don't find it shocking either, although I would have done some sort of sleep training with DS3 had he not been sleeping through by the time he was 1.
I personally couldn't cope (especially once I was on my own) with interupted nights. However many parents cope fine. My BF's DD (4 1/2yrs old) has only recently stopped getting up and going into their bed in the early hours of the morning. She was fine with that.
As long as everyone is happy I'm not sure what the problem is. Obviously if the parents are unhappy with the situation that's totally different kettle of fish.
Mawbroon - I had a "no terrible two's child" - he turned into a tempestuous 3yr old and is not a stroppy 8yr old