Going up is costly, coming down isn't (as much). I'm no rocket scientist, but I guess it would still be possible to have large heat-resistant/disposable wrappers around a large gunk of asteroid rock and send it crashing into the ocean, or land by a simple parachute.
There's a great idea I read once (can't remember the book), where space mining smelters inject nitrogen into the molten metal creating a foam. This drastically changes the thermal and density characteristics of the material and allows tons of the material to be dropped from orbit into the ocean without causing an accidental extinction event.
A better question is that this seems to only make economic sense for relatively rare materials, but the sudden flood of material would lower their market price potentially pushing them too low to be economically viable to support space mining.
