I believe it's a courteousy. Especially when candidates prepared for the job. The HR did ask to send a few documents over. I mean, if not interested, why bother calling the candidates? And when a candidate follow up with you, it means that the candidate cared for it, have a courteousy to reply instead of staying silent? It's not that hard. It's not like they're working until 1-2am like auditors. Try putting themselves in candidate shoes, will they be feeling good if no one replying to a follow up email? I believe the recruiter would change their job, go for interview and would love to be treated the same way ....