It's quite a matter of Physics... Imagine you're a real person on a real sub. Now imagine you're in the control room, and raise the periscope. You take a look through the optics, and when you're done, you want the periscope down into its housing again, but WITHOUT taking your eye off the periscope. Would you really be able to keep looking or stand properly?
As for the attack periscope, since it is a static optics plataform (per real design), you raise/lower the tube, but not the plataform. It spins with you, but it doesn't go up or down, neither do you, it's always in the same vertical position. That's why it can go down while looking through it.
Cheers
