It's a typical example to CONVENIENCE. It's convenient to re-send the message. No one serious of privacy would ever use Facebook / WhatsApp. So the title is

It's a typical example to CONVENIENCE. It's convenient to re-send the message. No one serious of privacy would ever use Facebook / WhatsApp. So the title is a click-bait. The decision behind re-sending is based purely on convenience and cost-benefit analysis. Actually I think they should display a notification / popup / warning whatever.