Worldly Thoughts

  Just because someone has 30% of the internet, they still have an
  interest in connecting their 30% of the net to .1% of the net, no?

Because they can already hit that .1% of the 'net already without
peering with that smaller provider through that smaller provider's
transit provider and peering:

1) Enables them to screw you up a little more than they otherwise could
2) Provides them with free 'transit' across your backbone (even if it's
   to/from your customers) if the peer isn't multiple-exchange-point
   connected to some of the exchanges you are.
3) Has those political 'what is a peer' problems also.