Well if you are going to talk of the OP, then you should be aware that the topic was created on: 06 May 2010, so the reference you make isn't about the OP but Ansatsuken.
What hasn't been asked of Ansatsuken is, have you modified the file system shield settings as it shouldn't be scanning the USB only files that are active/being accessed ?
So if avast is scanning something on the USB, it is because something else is accessing them, what that is is the $64,000 question.
I honestly don't believe that removing the USB drive without first having used the eject USB is wise, even if it is configured for "Quick removal," whatever that might be.
If windows is doing delayed writes, then that could be what is accessing the drive then that could be a problem which could result in data loss and a scan being in progress at the time of pulling USB. Now if the configured for "Quick removal" means that windows doesn't do delayed writes, this 'theoretically' shouldn't be an issue.