Out here, especially with social sites, RSVP is taking on another meaning entirely. Those "slots" are fictitious (just like airlines overselling seats) and assuming attendance numbers will be what something like Meetup says they are is irrational. I usually factor in a 40% attendance rate - nothing wrong with that if it's more, but it's unlikely to be 100%, unless one holds an irrational belief their event is going to net more than another.
That's not to say it doesn't make planning for the event a bit more challenging, but I seriously doubt there anything anyone can do about it other than charging for the event, which may not be desirable.