For Macs, there is an important difference between a Forward Delete and a Backward Delete.
The Forward Delete ⌦ key is also typically just labeled Delete or Del and is the same as on Windows keyboards. It deletes the character ahead of the cursor and is only found on full Apple keyboards.
The Backward Delete ⌫ key is usually known as Backspace and can be found on all Mac keyboards, even the small ones. Pressing it removes the character left of the cursor. To achieve a Forward Delete, one has to hold fn and press ⌫.
Now that this is clear, here's my interpretation of things. In fact, the Human Interface Guidelines do not mention the intended behavior of the delete keys and their modifiers. They only give recommendations for the usage of modifier keys in general. This explains the lack of consistency in your examples.
My observations:
The most commonly used key for deleting elements that can be selected in the user interface is Backward Delete ⌫ since it's available without a modifier on all Apple keyboards. This applies to songs on a playlist, graphics in an image editor, deleting text boxes in Pages, etc.
The Forward Delete ⌦ is mostly used for the same actions, so you can actually use both in an interchangable way for most applications, be it Numbers, Excel, iTunes, Photoshop, etc.
The combination ⌘⌫ is intended for actions that might have an immediate (unwanted) effect. One could indeed map trashing files and folders to a simple ⌫, but this could lead to accidental deletions. Finder uses the modifier as a safeguard. This also explains why iTunes will only delete a song without confirmation if you press the ⌘ modifier.
For what it's worth, I've rarely seen the latter combination used other than in Finder and iTunes. Apple Mail will map it for convenience, but this combination doesn't exist in most other applications and I'm sure you won't find many that accept this shortcut or map it to any useful action.
