Enabling for read-only in the general case simply wouldn't be possible. The device would have to know every USB protocol that could possibly be spoken and which commands are for reading and which for writing (and what to do with commands that do both).

You could maybe make now that only worked for USB storage devices and only allowed reading, but it would likely be complex and have other downsides (lack of performance and compatability issues probably) that would make it not worth it.