Well, I don't exactly have the words to explain this but here is the possible fool proof explanation. The island only took back those people who were in the original Oceanic flight 815 in their original state. Ben and Frank were never in the flight. And Sun gave birth to a baby off the island and earlier when she landed on the island she wasn't a mother or was pregnant. You guys are forgetting Locke also. He too didnt make it through to the 1970's as he was alive when he came to the island last time around. But this time he was dead. So the island did heal him(Brought him back to life) like it does to everybody but the island didn't throw him back in the 1970's.
Hopefully this explanation made sense.
P.S. What makes everybody so sure that when everybody initially landed on the island(As in when Oceanic 815 crashed) they were in sync with the world time. I am sure they were not. It was hinted to us in Season 4.