Perhaps I was too succinct in the point I was trying to make.

The reason we don't punish people for retracting wrongful allegations is because we want people to retract wrongful allegations.

If you're in jail on a false rape claim, you'd rather the accuser went free and you were released; than that you stayed in jail because the accuser didn't want to take your place.

Of course this needs to be done with care because retraction shouldn't be a reliable get-out-of-jail card if you use false accusations as a normal way of conducting business ;)