The problem here is that people don't understand the "usb scanning part".
"Scanning on connecting" is what avast does, but that doesn't mean it will scan everything on the usb device.
It only scans for certain things (behavior).
As Bob already said, things will be scanned on access (depending on the users settings).
Think about it...
Let's connect a 16 TB external hard-drive (yes they do exist) that is filled with files...
Do you really want to wait hours (or perhaps even days) before you can use it ?