It is... and it isn't.

When you speak with the immigration person, that person will determine if you will be subject to a search when you get to the 'customs' person. They then write something on your customs/immigration card and when you hand that in to the customs person, they will tell you to proceed to the exit, or be subjected to a manual search.

I would assume the agent at the 'custom's' post also has the authority to send you to a manual search if they suspect something, but every time I've beed searched, they looked at the card and said: Wait... you need to go here.