Aha, just found this in Config_Device_Changes.sh
# Add proprietary video card Device and drivers if needed
# TODO: allow user to express his/her will in using the open source driver if he/she so desires
OK, no problem. It's a todo
Looks like I just need to comment the line that says
radeon|ati) PackageIsInstalled fglrx-driver && VideoDriver="fglrx" ;;
from Utils.sh