Well, without a notice they would just be left wondering, thinkI concur with Lena. There are malcontents for which that's the better option. Any kind of response triggers them into renewed efforts.
Banned members are not necessarily 'bad people' who don't deserve anyI think that they are 'bad people', in the eyes of the group that banned them.
The fact that the moderators believed banning was necessary likely means that they were dealing with someone that is already hostile and attempting to get around the moderator's decisions. Or is someone like a spammer who doesn't care in the least about the group's purpose and rules.
The "non-bad people", who merely don't belong in a particular group, would likely get the hint at simple removal from the group.
I think they deserve the information.The moderators are free to reach out to deserving people; and probably wouldn't have needed to ban them. If the mechanism informs banned people automatically then that discretion is removed from the moderator's control.