Avast Software has for some considerable time refused to have this one click option to Allow, as it is just too prone to accidentally allowing a virus to run.
They have to consider the 180+ million users when considering options like this and air on the side of safety over convenience for the few. That is why they have gone down the deliberate addition to an exclusion list.
Allowing some means of circumventing this, registry entry or otherwise could possible be introducing a security hole, that could be exploited.