In Star Trek V: The Final Frontier, Sybok captures the Enterprise and confines Kirk, Spock and Bones to the brig.

Inside the brig there is sign on the wall in the background that reads

"Do not use while in spaceport"

Although at this point its not clear what the sign relates to.

Shortly afterward, Kirk pushes a round metal button under the sign and a seat slides out of the wall.

Why exactly are they not allowed to use this seat while in spaceport? What could possibly happen? Sorry I couldn't find an image of this sign.