Hi tibos,
Yes, you can see all the categories list in the dropdown but if you've created K2 user group and assigned them to categories, and assign each user to a user group, you can see the list of all categories but you can't select the categories that the user is not assigned to.
Example:
I create a user group Spanish and assign category - mi gato puede cantar
You have a user named - cat entrenador and you assign the K2 User Group - Spanish to him or her.
When they add article in the front page, he/she/heshe :) can see all the categories but only can select to add articles in the mi gato puede cantar category
K2 Rocks \m/