Hi John,
I won't say it's a fault, I believe it's design in that way by keeping some thoughts in mind.
Check the Hangman1212 comment in the following idea which can be one of the reason:
https://www-secure.symantec.com/connect/idea/automatically-demand-avas-scan-usb-devices-when-plugged
" For me, this is useful if the total size of the USB drive would be 8GB or less (maybe 4GB) and it has about 30 to 50% space taken.
Imagine if you insert a USB flash drive with 32GB or greater (maybe a USB external hard drive), and you occupied almost the whole disk.
Varying greatly on how you configured your scanning, would you rather wait for SEP to finish the scanning before you can use it?
Just what I 've remembered from someone from a Symantec representative here in our place"