(This isn't related to Weed's case in particular, because Andrew's apparently explicitly mentioned only the removal of mod's Weed invited, not all mods.)
When we ask for mod removals for inactivity, we usually mean new invitees who aren't working. And more importantly, we never mean old mods that another owner invited.
Each mod being their inviting owner's responsibility is a very old rule. This achieves two things:
1. Every owner has limited responsibility. Of course they can scour each mod's activities and take action, but their own invites are their only immediate responsibility.
2. When another owner's mod breaks the rule, we can rub it in the other owner's face (cool points gained).
P.S. All of this sounds made up but it's not. It's consistent with the stream's other very old but more commonly known rule about indefinite bans and unbans, which goes somewhat like,
the owner who banned a user indefinitely must be the one to unban the user.
