The death of player input

Player input. It is what keeps the 'game' part in rpgs. However, since beginning of rpg campaigns, there have been instances where player input has been taken away or has had no effect. Most roleplayers already know part of what I'm talking about, but there is another part that some people ignore.

Railroading. It has many definitions, but it generally means one player (GM included) taking away the choice, or option of choice, from another player. The party comes to a fork in a dungeon, and the DM describes them taking the left path (or doesn't describe the fork until later, when he mentions they took the left path). This is classic railroading. Not all railroading is bad, but nearly all railroading that is discussed as railroading is bad. The players had no input this matter. This is usually easy to notice.

Illusionism. Often overlooked as harmless, it is defined as the presentation of a choice that has no meaning or where there is only one outcome. The most obvious reason why most people brush this off to the side as harmless, is because it is can be very difficult to detect. Imagine three cups and a ball. The ball is placed under one of the cups so it cannot be seen and the cups are shuffled. You are instructed to pick the cup that has the ball under it. Now imagine that, no matter how closely you watched the cups, the cup you choose will either have the ball or not have the ball based upon what the GM chooses to have happen. That just sounds wrong to me.

In short, I think both railroading and illusionism are the same thing, but are accomplished at a different layer. Railroading and illusionism both kill player input. It doesn't matter if the party always takes the path on the right, the party went left this time (railroading). It doesn't matter that you only got a 6 on your bluff check, he still believes you (illusionism). Why is that part still 'played' if player input doesn't matter?