Because you cast Quick Spells not Fast Spells with the Quick cast marker.
Hi Shadow,
Thanks for the input on this one. I agree that in one way, it seems obvious to call it a Quick Cast as it allows the Casting of Quick Spells. However, it's possible to cast quick spells at other times too. In my own experience, it would have helped to separate out the clearly different 'Quick Cast' event from the normal turn flow by simply calling it something different.
Reading a couple of replies on BGG, it seems as though a couple of players at least have used alternative terminology to describe Quick Cast in order to avoid confusion over multiple 'Quicks'. They used terms such as small spell, little spell, free spell, etc.
I'm just keen to get as many people as possible playing this game, so anything that makes it easier is a good thing in my book.