Not sure why the request would be considered unusual. The app is a cloud backup app that monitors folders for changes. If the app crashes the folders are no longer monitored. It is important that the app can be resilient enough to survive a crash, restart and continue monitoring folders. Of course if the end user wants to quit the app it should be able to , but if the app crashes for any reason, like 100 % of app do crash at one point or another, it should have the ability to start again. I am not sure why a sandboxed app would not also have the opportunity to take advantage of toolsets to do this and be against the guides of App Review.

Its for the Mac App Store "login at startup" = Open at login