Go back to basic definition:
Positive Relief valve prevent an over pressure within the fuselage. Most are set to open some where around 8.5 psi differential and fully open at something north of 9.0 psi.
Negative Relief valve are to prevent a lower pressure within the fuselage than atmospheric pressure around the fuselage.
When a door or escape hatch is opened, to much positive pressure with in the fuselage would/could prevent this door/hatch from being opened. With a negative pressure within the fuselage the door/hatch would blow in causing possible personal injure.