This message is setup to appear once a 30 days if there is lot of clutter to clean.
Does it appear more frequently?
Doesn't Avast classify some other software with unexpected pop-up ads as PUAs and warn the user before installing?
I haven't kept track of how often Avast's advertisement pops up, but even once per install would be too often once a user pays for Avast. It's an advertisement. That means it's designed to make the user itchy and acquisitive. Some people's reactions to this will range from insult to moral objection. We don't want it in a paid product for the same reason that we buy TV shows on iTunes instead of watching them on cable.
The product it advertises is also snake oil in part, or at least it gives misleading advice. Blasting log files will affect people's experiences when they need to troubleshoot or work with Apple support. But, much worse, wiping out ~/Library/Caches is counterproductive on a machine with plenty of free storage and an SSD. Fragmentation or seek times aren't an issue. Space isn't an issue. Why would one aggressively purge data that one then has to wait to download again? Why should a user slow down his computer just to make his drive's free space temporarily look larger, even if he hadn't been tricked into paying for that privilege?