Every feature has to be written and tested before it can be released. If VBA-enabled documents were disabled but the document editable, that means that WP needs to be able to save documents with VBA macros in, otherwise you end up with customers complaining that when they get sent a document, they fill it out and email it back, all of the macros are missing.
Disabling editing when there is stuff in the document you're not sure you can preserve over an edit is less work than enabling edits and potentially corrupting the file or throwing away information in the document that customers might really want.
Which is not the same as:
"What feature should they have not implemented in order to spend time coding, testing and security checking VBA on Windows Phone"
"Should Microsoft introduce VBA into the next version for the small-ish numbers of people that want it, or should they add something else, like support for older versions of Excel documents?"
Do you really think preserving parts of a file that the app doesn't support is the same scope as adding VBA support?
Isn't the very nature of Office Open XML Format is to allow support for different versions of Office apps to with different feature sets to work on the same file without rendering it useless? It certainly isn't free to implement but it isn't near the scope of adding VBA support to the web and WP versions.
So EvilD have you settled on a scope yet or are all issues not raised by you nor kettch bad ideas because they require such things as development time and testing -- like any other feature?