I asked my instructor at the time why there were no limits and he didn't really know. Yet every fixed wing aircraft, even if all it has is a 65HP engine, has published G limits.
All helos are designed to cope with a 60 degree bank turn, which is 2g, plus a gust factor and a bit for mum and the kids - usually set at 2.7 g, so their accepted teetering head limits are
+1 to +2g, allowing the fudge factor to be set at 2.7. But that is when you can expect the head to pop off. The lower limit, as shown, is +1, meaning it ain't designed for nose-overs that get less than 1g, and certainly not negative g.
Stronger machines with a rigid head like the BK117 have a published limit of
-1 to +3.5, so there is still a fudge factor above that one.