Is it normal to see bandwidth utilization on the idle unit?

Yes, it is normal although the idle unit is not handling network traffic. Utilization is reported because network statistics are replicated at intervals from the active unit to the idle unit. This replication process prepares the idle device for a possible failover and retains these statistics in the event of a unit failure. 

Please note that the "Primary" and "Secondary" unit labels do not affect the process as whichever device has the "Active" status will replicate to the device in the "Idle" status.