Tbh that practice seems to be on par with any social network standard out there. You get stuff, even for free, but the real product is still your data. If you don't want anyone to access that, you can't use these things.
Members of groups don't pay. And it's their data that is valuable, since it comes in bulks. Noone would care if it was just the data of the paying group organisers.
There is a value in being able to spam via an already verified list.
If that value is more than a dump and run on the Meetup expense, there is an obvious arbitrage opportunity. Capitalism all but guarantees others will be able to leverage this along with the less obvious opportunities.