Cascading Failures Could Crash the Global Internet

The paper is avaibable on the Los Alamos site free of charge:

The model proposed makes several assumptions. My question is about:

   Many real-world networks are heterogeneous and as such are expected to
   undergo large-scale cascades if some vital nodes are attacked.

on page 3. I do not get the basis for this assumption. So any help for a 60's
educated math major would be appreciated.