If you know they've got an infection then I agree that's a problem and medical ethics would suggest treating it with antibiotics if available. But, they didn't know she had an infection.

You're assuming (and indeed US medicine seems to assume this everywhere) that since there don't usually seem to be major negative side effects from antibiotics they're harmless. However we are now confident they're not - lots of interesting things live inside us and whether or not they can cope with antibiotics varies, we're an ecosystem and so this intervention is a massive change to that ecosystem, and while it will usually be justifiable if we know there's an infection if we don't know that's now a gamble.