When an owner of a meetup decides to stop doing so – the ownership is offered to other members of the meetup group. This allows the group to continue even if the owner disengages. But yes, it does also allow the new owner to gain access to a complete list of members in the meetup group (since they are now the admin of the group)

It's a bit messed up, but it's been that way for a long time.

Meetup.com will first offer a group whose organizer had stepped down to the members of the group. They will not shut down a group easily.

I disagree that they shared emails since I do not think organizers have access you that data.