Well, to be fair, adding a softener before the RO membrane will likely raise the TDS out of the RO because calcium and magnesium are actually easier for the RO to remove than is sodium. So that isn't the reason to put the softener first.
As Joel indicates, the reason many suggest having it before the RO is that it reduces the calcium and magnesium carbonate precipitation onto the RO membrane (often called fouling or scale formation) and so the water softener can extend the lifetime of the RO membrane before it is too clogged to be useful.