People skipping the RSVP and just showing up "unofficially" is already a problem with Meetup. This is going to create the exact wrong incentive for that. Next up: movie theaters that charge $2 to anyone who doesn't talk during a movie, gyms that charge $2 if you put dumbbells back on the rack instead leaving them strewn around the floor, traffic cops giving tickets for properly using your turn signal, ...
But in that situation, attendees paying meetup is absurd but in just about any situation, attendees paying meetup is absurd. Meetup already has an option to have attendees to pay the organizer for attending. They might consider that for the events where this option isn't used, it is realistically because this could not work, you can't get money from people for that stuff.