I learned long ago that group owners are very possessive of theirI wouldn't be concerned if the notice went out by email to all users, from a general groups.io address. I just wouldn't want it to appear to my members as a posting in my groups (had the group subject tag and group footers).
For another thing, posting in all the groups would mean that someone who's a member of several groups would get several copies of the notice.
That's just me though. I've seen in Y!Groups that sometimes members don't recognize the distinction between notices from the service and notices/messages from the group. And accordingly some group managers are touchy about messages from the service as well.