Rereading Half-Blood Prince

A question has occurred to me. It occurred to me before, but I forgot ;P

How come house-elves can Apparate and Disapparate within Hogwarts? And to and from Hogwarts, too, I’d expect; that’s probably how Kreacher got there. Is this ability part of the reason why they’re endentured servants/slaves? In the distant past, did wizards think their unfettered Apparition access was too dangerous?

Did I miss an explanation of this somewhere?