The important point here is that it's not just the Chrome you download from web - it's been modified, e.g. the add-ons have been removed, to make it suitable for the SafeZone (safer).
Maybe it will be possible to run custom browsers inside the SafeZone in the future, but it's not possible now - and even if it were, not providing a safe default in avast! installation would be rather... unfortunate. What's the point of using the SafeZone if your browser is vulnerable, or even already compromised?