They know a great segment of the voters who a) turn out in the primaries and b) who might tend to vote for Cain would be moral conservatives, those who would be disgusted and would drop their support of Cain ASAP if the "charges" were somehow shown to be true. So, what do they do? They pick and issue such as this (actually a dead, non issue), to strike at the core of his base, sow fear, distrust and confusion. They would not have chosen lets say back taxes, or military service, or something like that. They had to pick a much deeper moral issue to sew their dark deed. In this sense, they are much like Satan, the Grand Accuser.
This is how they operate. It MIGHT be from one of the RINO camps, I am inclined to think that it comes from the DNC and Obama folks just by its nature. It has been well cultivated and sitting in waiting to be released at the opportune time. They do not want to run against Cain, so this might be the best time to whittle him down a bit. Well, we will know soon, because the other GOP candidates are going to have to say something about this one way or another in the current news cycle. Watch what they say very closely. (Ron Paul already did and changed the subject and said it was a non-issue, at least he had the decency to do that.)