Whilst the trusted sites is controlled by avast - e.g. you can't just add sites to the list - so they would have to be major sites of a known status. Should one or more become hacked/infected avast would have to A) monitor this and B) remove them of the list.
I'm a trusting sort NOT and I want all sites that I visit to be scanned.
You only have to read the papers and other media to see major sites getting hacked, etc. etc.
The benefit of not scanning a very limited number of sites considered trusted (at this time) would also be very limited, you may not even visit some of them.