I think people could be happy because they don't know what it means. It's a general feeling when people have a baby. My friends were excited when I had mine but none of us knew what it all meant until it happened. Plus unless you have been there what do you say? I think the comments are fairly normal responses. I received similar comments 'congrats, great news, wow that's little and early' to name a few.

Dont be hard on them. Maybe offer your help. Write something on the fb status saying 'congrats on the baby, hope everything is ok, having had a premmie I know what's in store and the scary ride so if there's anything I can do to help your cousin let me know'.