That's what happens when you use the primary Install button option I suspect.
With updates/new installs ALWAYS use the Custom option with any software.
The reason for the shame should be obvious.
If this is really happening when the AVAST install box is unticked (BTW deep shame on AVAST for pre-ticking it, one of the sneakiest tricks in the book) or even when opting out of the CCleaner Pro Trial then AVAST should be vilified and held to account.
Many people would assume that opting out of the CCleaner Pro trial also meant not wanting AVAST either. Obviously that AVAST box must be unticked whatever option you choose.
Dark practice and they'll be damned for it in all the PC magazines and software review web sites until that is changed.