You can stop control snatch by holding the controls tightly. But if you didn't, the elevator / rudder would probably be damaged quite badly, as the C152 control stops are very small and very close to the hinge line. Perhaps Mr Cessna is just playing safe with what is after all only a semi-aerobatic aircraft?
The 172 on the other hand is not aerobatic at all as far as I know, although it is spinnable.
To answer the original post, although the transition from up to down is approx 0g, the pendulum effect after transition to vertical down can give ±2g in my limited experience. Not enough to hurt a 152. Can't say it makes me feel great though...