I see how and why you think it was weird, but ultimately i don't think it was. I think its more about how everyone uses Facebook a bit differently.
For example, I am only FB friends with people I know in real life, and I tend to use my feed as a sort of personal news line - if I have news I'd like to share I post, and I read it to get news on people I my life.
But my DH for example, has several hundred FB friends, some are just friend-of-a-friend people who he has never met, etc. He uses Facebook to promote his bands and his friend's band's shows, and posts articles he finds interesting, etc.
So for me, I probably wouldn't post a friend outing as something public, any public event would be specifically billed as such, like a "Community Earth Day Event!" But my DH will post stuff on upcoming shows, or comment on other people's events to get the event to show up in people's newsfeeds, as a way to promote the event. Sure the bands invite all their friends and fans, but they are happy - eager in fact - to have new faces come, so these public events are truly public.
Someone who is used to seeing public events that are posted as public specifically to get more then just the invited audience are not going to intuitively realize that other people might post events that are actually just personal outings between a small group of specific invitees.