What a shame - I can't understand why someone would take these registers. I really don't think it's because people see it as trash on the mountain - they clearly don't seem to care about leaving their wag bags behind. It's also not a piece that would make a good souvenir - the point of a souvenir is to have something from a trip you can show others. You certainly can't show a stolen summit register to others, unless you enjoy being a thief and bragging about it to your friends.
I just don't get it.