I've been using Avast for years, usually apply the updates within a Google or two of being notified they're available, and have never seen an "opt-in by default" part of the install, except for a year or two ago, when Google Chrome was offered (and pre-ticked) as part of the Avast Free package, on initial install.
Certainly the update routines tend to lead one toward trying the full (and paid) alternatives, as the section to be ticked in order to keep the free version are a little less blingy than the other sections, but that's to be expected.