I don't know why I'm so curious about this issue (maybe it's because my 20th is in the works right now). VM, can you tell us a little more about the conversation where you asked for clarification for an in-arms baby? I do think part of this could be the organizers just really not knowing what a "sling baby" can be like. For some people an infant comes with gear, a pack-in-play, a bucket seat, a huge bag, bottles to be warmed, diapers to be changed and etc. It may be that this is what they're picturing, which I agree is not appropriate for an evening event.
My experience with group events is that asking for permission can put the organizers/admins in a tricky position. As we can see on this thread, there are people who know people who get very bent out of shape when a request is adjusted to fit personal needs. I think that's a shame but that may be where they're coming from. Perhaps they already said "no" to the parent of an older child or perhaps they know a very child-centric family with an infant that they have told can't come. Something like that.
I see this issue as one that's super interesting in terms of expectations for acceptance. I do think it's important that adults learn to deal with stuff not being just how they want it - but to me that includes both scenarios: both a mother showing up at an event with an in-arms baby (because, yes, whether this is the norm is obviously dependent on where you live) AND accepting that some people are uncool with that. I think if we all live our lives with a bit of flexibility and take our turns when we feel it's important to us and always give others the benefit of the doubt, life will be pretty good for us. ;-)
That said said, had you not asked, and just shown up, and you really do have a sling baby that is a good candidate for an event like this my guess is that the organizers would hardly notice and the people who got sitters are happy they left their infants because they had a nice time without and you had a better time with and you all have a great time reconnecting. That's what I would expect to happen based on my life-experience.