I'm a meetup organizer and while I haven't sent out any messages like this myself, I understand the reasoning. We have to pay dues to run the group and you pay more if you have more members. There's also a problem with people RSVPing to events and then not showing up-- which costs us money and sometimes makes the activities less enjoyable (because you need a certain number for participation). I've probably personally spent about $300 that were wasted because I had to buy supplies or pay in advance for tickets for people who no-showed or cancelled day of. I haven't personally made a big deal out of those things because I've also been a member who has had to cancel day-of due to illness, so I can understand both sides.