PUPs are detected by default in the File System Shield (FSS) and in the right click scan by ashQuick.exe.
It is only in the Quick and Full System scans where you can't select it, Sensitivity, PUP and suspicious files options are greyed out. If you want the avast on-demand scans to check for PUPS you would need to create a Custom scan and select the option.
Given the FSS covers this if they are run, I see little point in bothering with this on an on-demand scan as they are inert/dormant at the time of the on-demand scan. Should they become active, e.g. run then the FSS would first scan and presumably alert.