You don't really need to restrict drone riders just make some rules which describe why sane people don't *choose* to do it.
Imagine riding inside what's effectively a big donorcycle motorcycle that's getting shot at with guns. First, it's a drone, so you're probably going to take a hit every time the drone gets tagged (drone get defense, but only resistance for you!). Second, people are still susceptible to stun damage, so getting your drone armor rocked (but not penetrated) might not phase the drone, but being inside that coconut won't be a fun experience. Just imagine electrical damage on the drone! I'll have seconds of Baked Rigger please! Then you'll take damage when the drone crashes which should happen pretty easy for the poor stats that were mentioned and let's hope you were flying either in the air, or down the highway at the time.
The point is that drones are not vehicle and cannot offer the same protection. That's not a crazy assumption to build into the rules. While you might be able to stuff a body inside one, it's perhaps not the wisest thing to do for anything combat related. And/or you could always just make it so expensive that it's a poor choice mechanically.
Maybe an armored Rigger Cocoon is too heavy for a drone (and I think it should have cost more slots anyway), or counts as drone armor for purposes of stat reduction but saying you can't do certain seemingly feasible things like rigging from a valkyrie module or backpedaling and not allowing them on drones anymore seems like a bad idea.
