Must be nice... I've been at 30 ft and it was so dark, I couldn't see any
light when I looked upwards (yeah, it was daylight)...

Its merely a part of their choice of definitions.   At 130ft, you're
allowed zero penetration as per the first (penetration+depth<130) rule,
so that's simply repeating an existing limit.  At 70ft depth, this is
simply choosing to be a little more conservative.

We've all had dives where there was "enough" light at depth X.  Ditto
for some shallower dives (probably up North) that were darker than a
Caribbean night dive and needed a light even at high noon.

But it doesn't matter.  What does matter is that the caving agencies of
the NACD and the NSS-CDS both have defined their dividing line between
cavern and cave.  They're reasonably similar too.  They obviously use
them as an aid in determining appropriate training for each progression.    
And it was sufficient to show that Bob's accusation that I had invented
a new definition was yet another falsehood.  

FWIW, I would expect that a lot of us probably have crossed these
definitions of ow-->cavern-->cave without having the appropriate formal
Specialty Certification Card, just like how there's been many OW-I
divers who go dive deeper than 60fsw without an AOW card.  I know I have.

-hh