Yeah, I think so, too. They aren't divorced, the kid wasn't raised by grandparents, or anything like that.
My husband says they're just sort of clueless, so that's probably it. I was thinking that the stationer would have caught it, but looking again at the invitation, I think maybe they printed them themselves. You can do so much at home now, sometimes it's hard to tell.
So what I learned is that if I am doing my own printing, I'll get someone very good to proofread before sending. It's so easy to forget stuff that is obvious to you and that, as a host, not a guest, you aren't looking for.
I think this is why sometimes people leave off last names in situations like Lowspark's or in the save the date cards I get sometimes. I think they want it to look informal. But they are looking at it from their own perspective -- self-expression, sort of like decorations -- rather than the recipients' perspective -- a source of information. So they focus on the color, font, etc., but forget to think through the content from the recipients' perspective. Not a big deal, rarely a problem (I did once have to call and ask a shower hostess who "Katy" was, and we got a save the date from a couple whose very common names we didn't recognize at all) and easily solved when it is, and never offensive or tacky. But based on our experiences, I would always include last names, and after this, hosts' names, at least on the envelope.