I have never seen this on any other mailing list.
Is it because the mailing list configuration sets "reply" to be the user who sent the email and not the list, and then it is easier to send an email to the list by starting a new thread as opposed to "reply all" and then having to edit out all the email addresses except amsat-bb?
If the information in the emails to this list is helpful to others who search google and find it in the archives, it will be harder to find if it's across multiple threads.
Additionally, not keeping things in thread makes it harder to follow a conversation and more difficult to manage email. If I am not interested in a particular thread I can just mark the whole thread read as opposed to two or three other threads.
If a reply to an original thread is truly a different subject then it is common practice to change the subject like so:
How do I search the archives? (was: Why do people start new threads when replying to a post on amsat-bb?)
73,
John, KG4AKV