I belong to several meetup groups and have seen messages like this from time to time. Haven't received any addressed specifically to me - they were all mass mailings. I get the impression that this is a fairly common practice for group organizers trying to trim down their member base to people who are truly interested (which tends, on average, to be a tiny percentage of their membership). One thing to remember is that Meetup charges organizers more per month if their group has more than 50 members. If an organizer wants to save money, it makes sense to whittle the numbers down to the active members only.

That being said, I think this woman should have given you more of a chance. You expressed interest even though you couldn't attend a particular event. To me, that says you're serious about the group and should be allowed to stay in. Apparently she sees it differently. Did she ever reply to the message you sent? If not - or if she did, and stuck to her original position - maybe she's just an unpleasant individual and not someone you'd want to deal with anyway.