They are scanning your boarding card and taking information they are not entitled to. As I stated earlier the reason for them wanting to scan a second time is to monitor numbers - it is nothing related to the passengers security and there is no legal obligation for any passenger to agree to this if they so wish. They can threaten as much as they like it will not stand up, and if it means denial of boarding then they are legally responsible.

We live in an environment where information security is key, they are potentially violating this - because you don't know what they are doing with this information after your boarding pass has been scanned.

From my point of view I politely decline, if they don't like it I call for the supervisor.