Isn't that a feature? Joking aside though, isn't the Styrofoam to give it a low density structure?

It's not a feature, its cost savings. You will never see good quality drones with styrofoams. The styrofoams makes drone lighter so that battery can be also cheap and manufacturer can claim bigger number for flight/charge. One advantage of styrofoams is that you can change it cheaply it breaks but in reality styrofoams makes drone much less stable because even small wind can now throw things off. It also makes it much more fragile.

Is there styrofoam on the Bebop other than the side guards? Those side guards are intended to be removed for outside flight.

light, cheap, strong... pick any two!