Why is 'ON' there?
off

Used to indicate movement away from a position ON

I took it off the table.
Come off the roof!

Question

Why is 'ON' there?
off

Used to indicate movement away from a position ON

I took it off the table. 
        Come off the roof!

Answer

It was ON the table. I took it OFF the table.
You are ON the roof. Come OFF the roof.
Something/somebody is already ON something, indicating movement from that position ON something requires teh preposition OFF.

Answer

"on" is normally opposed to "off"