Cross-posting to multiple lists is considered poor net-ettiquette. From another list:
"Cross-posting to the xyz-List and to another list, is generally counter-productive. No list's archive will contain the complete thread; cross-posts from non-subscribers continuing the thread will bounce; and xyz-List subscribers who are not also on the other list will only see half the thread, as will those on the other list who are not also reading xyz- List. Subscribers to both lists may see two copies of the message, annoying them as well."
Norm is famous for doing this, as are a few others. Few announcements and even fewer discussion topics are so important and urgent that this informal proscription against cross-posting should be violated.
Let's say there is a CA list and a SF list. One should assume that everyone knows the existence of these lists and subscribes to one or more as their interests dictate. Then, a poster should decide if his topic is of more interest to the CA list subscribers or the SF list subscribers, based upon the mission statements of each.
It is rude and condescending to assume that there are a large number of SF list subscribers who are interested in CA messages, but are too ignorant to subscribe to the CA list. It is rude and arrogant to assume that one's message is so important that it is OK to bombard every list with your all-important polemic. One should assume that the people on the SF list do not subscribe to the CA list because they are not interested in receiving those messages. Therefore, sending messages of a CA-nature to SF-list subscribers is similar to spam.
If someone from the SF list responds, only SF list subscribers will see the response. If Norm responds to that, he will no doubt send to all the lists again. This puts the SF-list responder at an unfair disadvantage in the debate, and the CA list subscribers will only see one-half the traffic - that originated by Norm.
Rarely, there is a rational reason to post to mutiple lists, such as an announcement being made to largely non-overlapping lists where the topic is expected to be of interest to both. When done, this should include an apology to those that are receiving multiple copies. Norm's messages, including the one I responded to, don't fall into this category, and have never to my recollection ever included an any apology.
Some list moderators will delete any post they see cross-posted on another list in order to protect their subscribers from the irresponsible behavior of a few over-exhuberant participants.