IIRC there are some macOS APIs that you can only access if your app runs out of /Applications. There are some features of an app called "Secretive" (an SSH agent that stores keys in the Secure Enclave) that only work if you have the app installed under /Applications (whereas I'd normally install it under ~/Applications).
1pass probably does this to ensure that people can't accidentally install the app the "wrong way" and break some features.