Who is unable to ride in the back seat? AND Why not?

"Can a passenger sit in the front seat of the taxicab if no room exists in the back seat?

From Driver Rule 54-15(h) (in PDF):
The Taxi and Limousine Commission's (TLC) rules state that a passenger who is unable to enter or ride in the passenger part of the taxicab must be permitted to occupy the front seat alongside the driver."

How is the partition supposed help when the passenger can always sit in front?