Hi 1207463,
This is called an Assembly Feature. And that is just the way it works, you're only able to mill/remove material from the assembly level, if you need to add material just edit the part and create the sketch in that part.
The reason behind this goes something like this: Imagine we have a part that we make and stock in inventory, but in some assemblies we add a hole and a kerf notch to the part. We don't want these modifications in every place the part is used, so placing the feature at the assembly level let's us have this flexibility.
Also just as a tip, you might want to use the words "assembly file" rather than "assembly drawing". Typically in Inventor assembly drawing refers to a drawing file (IDW or DWG) that details an assembly. Same goes for "part file" vs. "part drawing".Just a minor point, but it helps make your questions easier to understand.
I hope this helps.
Best of luck to you in all of your Inventor pursuits,
Curtis
http://inventortrenches.blogspot.com