Simple. The magnet stays on for a prolonged period of time to catch a ball, long enough to absorb the ball's momentum and bring it to a dead stop. However, when throwing a ball the magnet is turned on only for a fraction of a second, so this doesn't have time to happen. A good example is the shrunken head lock in Ripley's Believe it or Not. The ball is trapped, drops a couple of inches, and is then flung forward past the magnet. This is actually so reliable that it was implemented as a main game feature and replaces moving parts.