Lock in strategy? The strategy is to foster local community--nothing else. If it would work to not charge money they would. In fact, pretty much the only reason Meetup charges is to ensure organizers have "skin in the game" and as a result take it more seriously.

Meetup has a very well put together and well documented API. You can pretty much get all you need from that if you'd like to migrate away from Meetup, though, they do in fact make it hard to get email addresses to avoid spam.

But, you could easily write a simple OAuth app which authenticated a user, pulled their details and asked for an email address to migrate away.