I think it would be better if I explain about the reason behind it.
From our side, the Antivirus is associated with the offline antivirus. When you disable the offline antivirus, then it will unregister itself from antivirus protection. This unregistration from antivirus is mandatory, to let Windows Defender to take over the role of the offline antivirus. This works until Windows 8.1.
Anyway, when SecureAPlus offline Antivirus is disabled, actually it can't guarantee to perform the role of traditional antivirus any more, since it will rely heavily on internet access, that's why it is unregistered from antivirus section. This unregistration is also to allow other third party offline antivirus engine to take over the role. However, when the offline antivirus is disabled, SecureAPlus application whitelisting and Universal AV still functioning as protection for spyware and unwanted application.
Actually the problem that we are facing now is the same as all other third party antivirus compnaies. When you install any third party antivirus, Windows Defender will switch off by itself.