Taxes module ignores shipping flag on attribute features

I've stumbled across a problem with sales tax calculation when "apply tax to shippable items only" is selected.

It seems that the Taxes module is only checking the "shippable" flag at the main product level and doesn't take into account if a selected feature is flagged as "not shippable"

Here's my specific situation:
A record label based in California has to charge in-state sales tax on physical goods (CDs) but no tax on digital goods (downloads).

They are a "Music" product with the options of "CD" or "download":

Music Item <--- flagged as "shippable"....that way the "CD" option will trigger a shipping charge and sales tax.
- CD option <---No shipping flag available at this level (that's why the main product needs to be shippable)
- Download option
- File Download "feature" <--- flagged as "not shippable" which **should** mean no shipping or sales tax will be added.

So, here's the problem. If customer chooses the download option, they are not charged shipping, but for some reason they are still charged sales tax on the item....even though I have selected "apply tax to shippable items only" in the Taxes setup.

Maybe I'm missing something in my setup, but it seems like the shipping module understands something about the product that the the taxes module doesn't.

Any ideas? The only workaround I can see, is to have separate products for CDs & downloads....doubling the size of the catalog and confusing what should be a fairly straightforward shopping experience.