Because it's clear what the intended clarification of these experiments is, but less so for time travel. When the thought experiments are posed, the goal is not to find the answer to some question, but to understand the described situation, which might as well involve additionally specifying it.
The previous open thread has now exceeded 300 comments – new Open Thread posts may be made here.
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.