I appreciate the response.
Avast is not modifying the Windows operating system, therefore there should almost never be a real need to reboot the operating system when Avast is changed--the only exception would be if Avast was completely uninstalled.
If Avast modifies its own services, and the Avast developers really think the changes are so urgent that they cannot wait for the next user-controlled reboot, then Avast could restart those services without rebooting the operating system--silently, and without disrupting anything else going on. IMO Avast is rebooting the OS solely as a matter of convenience for itself, since it's an easier way to restart all of the various Avast services.
I would also note that, while Windows 10, by default, does not really shut down when a user tells it to (and this is user-configurable), Avast could read that bit somewhere and pop up a message informing the user that s/he should restart the computer when it is convenient. In any case, the specific experience that I described here--two forced reboots two days in a row--happened on a Win 7 machine, which, by default, actually shuts down when I tell it to shut down.
Anyway, I realize that I am tilting against a windmill here.