I remember reading somewhere (Tucker Abbott maybe) that forcefully
throwing the live beached shells back in the water may cause the muscle
to become separated from the shell. However, I have used that technique on
many an Oliva sayana and I have never seen any evidence that it was
harmful. However, in my experience, the shells whether Stromb, Olive,
or a Terebra, are usually on the beach for a reason (food, breeding, etc.).
Unless they were washed up by a storm, we may not be doing the mollusks a
favor by interfering with a natural event.  In the case of the Oliva,
it was either have them all collected and killed or save whatever was
possible.
Four beach renourishments later, that problem doesn't exist anymore as
there are no more Olives where there were formerly many thousands.

The Hastula cinera salleana are now swarming the N. E. Florida beaches
by the millions and can be found virtually everywhere on the beach.  It is
a yearly event where they congregate on the beach for breeding.  Throwing
them back into the surf wouldn't be doing them a favor.
http://www.mindspring.com/~strombus/terebra.htm

Bill F.