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.
That would be strange. The macros in .xlsx file is in seperate subfolder (XLSX format is zip file with folders of XML files). That means it's at least "at the surface" no implication to modify data and save the file again with macros leave there un-touched.
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.
In Office for Windows Desktop we can still edit data with macro disabled. Just display a line on top that macros will be always disabled on WP version would be enough. At least those want to edit can edit.
That's way better than turning your Office for WP to be ExcelViewer for all the documents that contains workflow related macros.
ExcelViewer for Windows Desktop sized 75MB. Even if you make it 5 times to cover other office document viewers, it's still much lighter than the current size of Office for WP.