We had a discussionaobout something like this awhile back and someone used the term summoning chip (I think). Anyway, the way it was related was every turn a player has a figurative summoning chip given by Game Mechanics that allows for 1 only Normal Summon (effects can grant you more), it is similar to the 1 and only 1 Attack chip that Game Mechanics give to Monsters. When You Summon (or a monster attacks) you use up that once per turn chip. Irregardless of whether or not the summon (or attack) gets negated, (unless another effect is granting you another), you have already spent your 1 chip. It is gone. You cannot do it again.
For Shien, it is an effect, not a Game Mechanic that says 1 card. Now it could be that Shien is continuously looking to stop a second activation. If a first activation is negated, then it is still looking to prevent a second, BUT another activation, it sees as still a first, because there has been no first. Then, it looks to prevent a second activation (again). Or, it could just be BKSS (or better wording in Japanese). IMHO