Why aren't the rescue swimmers trained to ride the hoist down?
They are, and they do... it just doesn't show up on TV very often. They don't free fall a very high percentage of the time, but if you get all your info from TV, you'd think that's all they do.
Free-falling comes from the fighter aircraft rescue mission, which is still part of the rescue swimmer's bag of tricks. It's still one of the best ways to rapidly approach a small life raft or someone encumbered by a parachute.