@oRx-Qx pirateradio
Permission groups are no "discussion groups" - You do not interact with other members directly, only with the source channel. By responding to the source channel, you chose to publish your response, so other readers of the source channel can read and quote your comment. I learned that a data flow embodying this logic is a specific thought through design of Hubzilla/Zot/Nomad - which is great (...that Zot has it - Not so great that its specific only to Zot, it should be standard...).

When you agree to the public or selected people sending you messages (which is always your choice), it is not your choice who else they adress in the same message by using sub-lists of their address book. You can leave a discussion group, but you can not leave an address book. You can, however, ask to be removed (mailing lists are required to offer that option) or block messages from its owner because he is abusing your address for spamming.

Therefore, it is wise that people from the same address book sub-list of the originating channel cannot contact you directly - only as a response to the originating channel. This is substantially different from "forums" or "group channels" which is a host of group discussion that you specifically join.

Regarding the icon, I would prefer not to lock the list members, but to hand them an access key (key symbol instead of lock). But that is fine tuning.