Agreed. No argument here.
I see the same thing. Not all sites are rated.
This behavior would be directly related to Avasts' business relationships with whatever browser company is involved, what legal agreements were made between them, what country, what overall restrictions legally placed on them, what region and language, and so on.
The program in question, Avast Online Security, has to be designed to take all these factors into account, and more, and still provide a modicum of ratings protection for free.
Even if a site is marked as green arrow, this does not mean it is always clean, just so you know. There may be a higher probability it is, but that is all we (you and I) would know. The converse is true of a site marked in red, it may have had malware removed, and has been properly updated and remedied, so it is now clean but still shows as red.
Point is, this is a guide and is meant to help us make better decisions on what site to visit.