Because when the same people in the group that attacked Palin decide they arent done, it is most unlikely that an all new group of PDSers comes out of nowhere to go after her kids with a fresh start. Back in a saner time we used to put blame where it was due, not search vainly for excuses and invent others when none could be found.
Those Palin family rumors were circulated by people who hated Romney just as they hated Palin: Andrew Sullivan, left-wing bloggers, etc.
There is a whole political world outside of the Republican party.