Is this to cheat me of my key?

EDIT: Looked at the wrong giveaway.

Are you sure that the key is for the correct game?

Could it have been an new user that didn't understand that they need to provide said game? Some new users make the GA's in the hopes that THEY will be the receivers of the game instead or that the site itself will provide it.

They may have the game marked as private using the new steam feature or the key you gave them is for another game that they do have and it's not the one the giveaway was created for.

In the giveaway page if you go to winners, there's an eye icon next to the key if they "viewed" the key. Is the icon there?