Possibly because people don't understand or know of any favourite device, I think we are going to need more information as to exactly where this is.
I personally have never heard of this favourite List you talk about. I have never seen a star symbol, and the only thing close to a star symbol is the settings Cog wheel).
I have 4 devices linked to my.avast.com account, aside from not seeing the purpose of setting a favourite device (surely you should know what is your favourite device)
EDIT: typo