Hi Jonathan - as in many questions like this it's because few, if any, people have asked for the ability to bulk edit VAT fields, the original request I think was or account codes. Here's the same question 4 years ago: https://secure.clearbooks.co.uk/community/ideas/1172/bulk-edit-vat-rate-on-unpaid-invoices-bills
CB will tend to be steered by ideas posted AND the votes they get, so you may want to add this as an "Idea" to see how many will vote for it (I will). I would make the point however that I think the failure is more at RB's end, where the data entry is supposed to take place, in not allowing VAT rates & classifications, if they did, CB would be able to update the integration.
On that point, you will need to ask for VAT type & rate to be editable because I have a feeling that a 0 in the Tax field in RB may be imported as "Exclude from VAT return" rate.
All the best