I guess they fear a lawsuit if something happens. You know, there are a lot of tourists who cannot read warning signs.
Or at least they pretend they cannot read them.
The park service's current approach is to open the falls during the winter when no one will swim in the pool due to the temperature. The problem with this particular spot is speed of the water and de-oxygenation, which reduces buoyancy to the point that even strong swimmers just sink. I too was pretty bummed when NPS first closed this, but the more I've thought about it the more I can understand.