that's essentially true (I think it's a bit trickier and more subtle than the way you expressed it) - but that's because it's the way the Law works - it's not because the RFU have decreed it to be so.
Danny Hearn broke his neck trying to tackle Ian Macrae, but everyone agreed it was just a very sad accident (the two are actually good friends these days). However in the cases of Ben Smoldon and Richard Vowles, the broken neck was deemed to be the referee's fault because he allowed unsafe scrummaging.