I suppose their argument would be that a belly pan alters the aerodynamics so could make the bike faster or less stable at speed so could increase the risk. At least they are referring to modifications and accessories which makes it seem more logical. Most of the things that are causing the complaints here are what most of us would think of as accessories (heated grips, crash bungs, Scottoilers, stickers, etc) rather than modifications. While they are not performance enhancing modifications, they are all things that may make a bike more attractive to a thief so increase the risk of theft.