Sticking a placard on something doesn't make law, any more than you sticking a no parking notice outside your house would do.
Where placarding is required it is generally called up in the C of A/Permit.
The POH forms part of the C of A.
The prohibition does not come about as a result of the placard, it comes about as a result of a condition in the C of A/Permit. The placard is there as a reminder.