One needs to consider how Microsoft set up Windows Defender (full-fledged anti-virus) on Windows 8:
1) It was included to guarantee all users would be protected from the outset... no one would receive a system without SOME antivirus present and running. At the same time,
2) It was programmed to be "the anti-virus of last resort"... meaning it would only be active if Windows determined no other antivirus was present, active, and current.
2a) If you, or an OEM on your behalf, install/enable another anti-virus program, Defender automatically disables itself; but
2b) When you uninstall another anti-virus... or if that anti-virus is deemed no longer up-to-date... then Defender automatically re-enables itself to offer you continual protection.
It's rather an ingenious idea --- to guarantee a user will ALWAYS be protected... while at the same time, deferring to the presence of a third-party antivirus supplied by you [or the OEM supplier]. In so doing, Microsoft was able to avoid issues of forcing/dictating a monopoly position in the anti-virus market.
One of the problems that you need to consider: even if you "disable" avast's run-time protection modules, its drivers and system files --- which were loaded during boot-up --- still remain intact. And these can potentially lead to a conflict with Defender [or any other second anti-virus running on the system] despite avast no longer being "active". Microsoft is actually looking out for you --- avoiding slowdowns and/or crashes --- by arranging things in this manner.
Hopefully, this may help you appreciate what otherwise would appear to be a "stupid" situation.