Just rebuilt this low mileage 1776cc motor that seized with about 1000miles on it.
The cause was a crimped rear bearing that locked onto the crankshaft
the shims were equal to or a fraction more than the end float you can see the abrasion on the mating face and 2 have welded them selves together with heat generated by friction and they will not pull apart easily.
The three shims are there to provide slip to save wear on the bearing thrust face so need to be free to move.

So if there had only been one shim or both were slightly thinner then they shouldn't have expanded through friction and seized ?

Sounds like the seizure has not damaged anything else but the shims - which is good news.

They weren't lying when they said there was no end float. 

Crushing the shims against the rear saddle bearing deforms the bearing which then seizes on the crankshaft locking the motor luckily it has not spun the bearing and damaged the case.