For certain events, for example "meeting invites" from other people, Apple does not give 3rd party developers "permissions" to edit/modify an event. Since we cannot change the time for the next snooze alert, we tell you that we cannot modify it and if you want to set a new reminder time, you have to edit the event thru Apple's Calendar app. They will allow you to change that alone, but only thru the app itself.
Hope that makes sense.