Maybe I'm confused here. For background, I thought that even in MWI some 'worlds' might not have conscious observers. Normally we can comfort ourselves with the thought that extremely low-amplitude configurations (like those in which ravenous pink teddy-bears spontaneously destroy all that we hold dear) might not cause anyone pain because they might lack the ability to support consciousness. (Obviously I'm ignoring Tegmark IV here.)
But surely every configuration of ones and zeros in the computer has equal amplitude. That would mean that if we 'observe' each bit, the world we then live in has the same amplitude as each of the horribly-suffering-simulations. On what grounds can we say that the latter don't happen?
In this construction every configuration of ones and zeros have equal amplitude, yes. However, most of them are nonsensical; the sum of the measures of meaningful worlds are very very close to zero.
Meanwhile, the sum of measures in this scenario where you exist is, well, 1.
That you see each of the nonsensical numbers with equally low probability doesn't matter. If you roll a d1000 and get 687, the chance of that was the same as 1; you still wouldn't expect to get 1. In the same way, you wouldn't expect to get any particular configuration, but you're effectively summing over all the nonsensical ones, and that sum is pretty close to 1.
From David Deutsch's The Beginning of Infinity:
I'm not so sure we have the computing power to "simulate a person," but suppose we did. (Perhaps we will soon.) How would you respond to this worry?