The Default Behaviour is not different from any other Behaviour. The only thing that differentiates it from the rest is that its activated by Remote Buddy when no other Behaviour seems to match the currently running applications. Like all other Behaviours, the Default Behaviour:

1) has a mapping table (Prefs > Mapping > Default Behaviour) - and its current mapping is displayed when you press the menu button as part of its menu. 
2) has a customizable menu (Prefs > Menu > Default Behaviour) and you can add any items to it, filling the - by default - empty menu. 
3) only has one menu item - "Hide menu" - when the behaviour is active and its menu has no entries 
4) has its menu shown when you press the "Menu" button´and you're using default settings.

I can't see how any of this qualifies as a bug. If anything, this is fully consistent with how all other Behaviours are handled, too.

As to how you can enter at the top level menu instead of the Default Behaviour's menu when pressing the menu button, you've already found out how to achieve this - by assigning an alternative menu action to the Menu button in the mapping table of the Default Behaviour.

Best regards, 
Felix Schwarz