Well even if it were to ask about creating a restore point before installation or uninstall, that doesn't impact on avast's ability/effectiveness to remove viruses.
Avast operates at a low level and can remove viruses without having to disable system restore. If it couldn't then all of your good restore points would be lost if system restore were disabled.
Should avast find a virus in a restore point it can also remove that, without having to disable system restore with the loss of good restore points.
I'm no fan of system restore as it isn't 100%, isn't a backup system and can have unexpected consequences, but it is better than nothing. It has been disabled for years on my system, but I have hard disk imaging in its place, you can't just decide to disable it.