Slime blocks won't launch players who are moving

If a player is walking atop a slime block as it is pushed upward by a piston, the player won't be launched. The player sticks to the surface of the slime block as if it were a regular block. This happens whether the slime block is pushed vertically or horizontally. It also doesn't matter whether the player is walking or just moving from momentum, as long as the player is moving in a direction perpendicular to the piston.